Hi, the first entry of my partition table is:

$ sudo hexdump -Cv -n 16 -s 446 /dev/sda
000001be  80 01 01 00 83 fe ff ff  3f 00 00 00 81 1c 20 03  |........?..... .|

(-Cv describe the output format, -n 16 asks for 16 bytes and -s 446 skips the first 446 bytes)
You can see that my first partition is a primary Linux partition and that this partition begin at sector 63 (see for example here for the structure of the partition table).
I would then expect that except for the first 63 sectors and the other partitions, /dev/sda1 and /dev/sda are exactly the same.

But this is not the case, the sector #2 of /dev/sda1 is not exactly the same as the sector #65 of /dev/sda (but they are very similar, only 16 bytes are different):

$ sudo hexdump -Cv -n 512 -s 65b /dev/sda
00008200  00 20 19 00 90 03 64 00  2d 00 05 00 5a 2f 56 00  |. ....d.-...Z/V.|
00008210  b6 b1 16 00 00 00 00 00  02 00 00 00 02 00 00 00  |................|
00008220  00 80 00 00 00 80 00 00  00 20 00 00 d8 38 ee 4c  |......... ...8.L|
00008230  9a 01 ef 4c 05 00 24 00  53 ef 01 00 01 00 00 00  |...L..$.S.......|
00008240  59 23 e9 4c 00 4e ed 00  00 00 00 00 01 00 00 00  |Y#.L.N..........|
00008250  00 00 00 00 0b 00 00 00  00 01 00 00 3c 00 00 00  |............<...|
00008260  42 02 00 00 7b 00 00 00  85 23 eb f2 71 67 44 f5  |B...{....#..qgD.|
00008270  bb 8f 6f f2 3a 59 ff 4d  55 62 75 6e 74 75 00 00  |..o.:Y.MUbuntu..|
00008280  00 00 00 00 00 00 00 00  2f 75 62 75 6e 74 75 00  |......../ubuntu.|
00008290  d8 3c df 5d 00 88 ff ff  52 d0 ef 1d 00 00 00 00  |.<.]....R.......|
000082a0  c0 40 51 b6 00 88 ff ff  00 4e c8 bb 00 88 ff ff  |[email protected]......|
000082b0  c0 f6 86 b8 00 88 ff ff  30 2e 0d a0 ff ff ff ff  |........0.......|
000082c0  38 3d df 5d 00 88 ff ff  00 00 00 00 00 00 fe 03  |8=.]............|
000082d0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
000082e0  08 00 00 00 00 00 00 00  00 00 00 00 8a 53 d3 0e  |.............S..|
000082f0  7c 7a 43 e4 8b fb ca e0  72 b7 fa c8 01 01 00 00  ||zC.....r.......|
00008300  00 00 00 00 00 00 00 00  16 4c 47 4b 0a f3 03 00  |.........LGK....|
00008310  04 00 00 00 00 00 00 00  00 00 00 00 fe 7f 00 00  |................|
00008320  24 b7 0c 00 fe 7f 00 00  01 00 00 00 22 37 0d 00  |$..........."7..|
00008330  ff 7f 00 00 01 00 00 00  23 37 0d 00 00 00 00 00  |........#7......|
00008340  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 08  |................|
00008350  00 00 00 00 00 00 00 00  00 00 00 00 1c 00 1c 00  |................|
00008360  01 00 00 00 e9 7f 00 00  00 00 00 00 00 00 00 00  |................|
00008370  00 00 00 00 04 00 00 00  9f 7d bb 00 00 00 00 00  |.........}......|
00008380  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
00008390  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
000083a0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
000083b0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
000083c0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
000083d0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
000083e0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
000083f0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|

versus

$ sudo hexdump -Cv -n 512 -s 2b /dev/sda1
00000400  00 20 19 00 90 03 64 00  2d 00 05 00 5a 2f 56 00  |. ....d.-...Z/V.|
00000410  b6 b1 16 00 00 00 00 00  02 00 00 00 02 00 00 00  |................|
00000420  00 80 00 00 00 80 00 00  00 20 00 00 df 76 ef 4c  |......... ...v.L|
00000430  df 76 ef 4c 06 00 24 00  53 ef 01 00 01 00 00 00  |.v.L..$.S.......|
00000440  59 23 e9 4c 00 4e ed 00  00 00 00 00 01 00 00 00  |Y#.L.N..........|
00000450  00 00 00 00 0b 00 00 00  00 01 00 00 3c 00 00 00  |............<...|
00000460  46 02 00 00 7b 00 00 00  85 23 eb f2 71 67 44 f5  |F...{....#..qgD.|
00000470  bb 8f 6f f2 3a 59 ff 4d  55 62 75 6e 74 75 00 00  |..o.:Y.MUbuntu..|
00000480  00 00 00 00 00 00 00 00  2f 75 62 75 6e 74 75 00  |......../ubuntu.|
00000490  d8 3c df 5d 00 88 ff ff  52 d0 ef 1d 00 00 00 00  |.<.]....R.......|
000004a0  c0 40 51 b6 00 88 ff ff  00 4e c8 bb 00 88 ff ff  |[email protected]......|
000004b0  c0 f6 86 b8 00 88 ff ff  30 2e 0d a0 ff ff ff ff  |........0.......|
000004c0  38 3d df 5d 00 88 ff ff  00 00 00 00 00 00 fe 03  |8=.]............|
000004d0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
000004e0  08 00 00 00 00 00 00 00  00 00 00 00 8a 53 d3 0e  |.............S..|
000004f0  7c 7a 43 e4 8b fb ca e0  72 b7 fa c8 01 01 00 00  ||zC.....r.......|
00000500  00 00 00 00 00 00 00 00  16 4c 47 4b 0a f3 03 00  |.........LGK....|
00000510  04 00 00 00 00 00 00 00  00 00 00 00 fe 7f 00 00  |................|
00000520  24 b7 0c 00 fe 7f 00 00  01 00 00 00 22 37 0d 00  |$..........."7..|
00000530  ff 7f 00 00 01 00 00 00  23 37 0d 00 00 00 00 00  |........#7......|
00000540  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 08  |................|
00000550  00 00 00 00 00 00 00 00  00 00 00 00 1c 00 1c 00  |................|
00000560  01 00 00 00 e9 7f 00 00  00 00 00 00 00 00 00 00  |................|
00000570  00 00 00 00 04 00 00 00  a3 7d bb 00 00 00 00 00  |.........}......|
00000580  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
00000590  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
000005a0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
000005b0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
000005c0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
000005d0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
000005e0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
000005f0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|

For example in the third line, there is a 8.L in the first hexdump and v.L in the second.

Why are there differences?