boot.img and the partition table

Hello everybody,

I'm wondering if any of you could give me a hint about a thing that is puzzling me:

MBR stays in the first 512Kb;
boot.img is exactly 512Kb (it makes sense);
but while using hexdump (or xxd) to inspect the first 512Kb of /dev/sda (my boot disk) and the boot.img I see different results on the partition's 64Kb part. The end signature is ok (55aa) as is the initial grub2 'eb63' default signature.

Should the results be the same? Is the partition part of the boot.img ignored while booting?

I was thinking that in case of a disaster (like having no partition table information) I could use a backup of the boot.img to figure out what the partition table was before the emergency has occured but it seems that the information is not there.

Thank you