System Backup vs Disk/Partition Backup

[Froggie]

Greetings!  While running both images from the Emergency disk (static disk being imaged), each one containing the EFI, OS & Recovery partitions (it's an UEFI System)... for a 26.4gB image, the System backup was 20,061 bytes SMALLER than the Disk/Partition backup of the same partition set.

Can you give me an idea why there would be any difference at all in those images...???  Thanks!