squashed then re-squashed give different size?

I extracted a firmware.bin using fmk mod kit and gave me 3 files: header.img , rootfs.img and footer.img now whenever I cat and repack all the files together in firmware2.bin again, it works and it upgrades the router. but when I unsquash the rootfs.img using this command unsquashfs rootfs.img into squashfs-root/ then I squash it again using mksquashfs rootfs-root/ squash_new.img -comp lzma -b 131072 "which it by the way the same compression method and block size as the original rootfs.img" but it gives me a less size comparing to the rootfs.img and the router gives me upgrade failed here are the sizes of the 2 files > squash_new.img (9,945,088 bytes) > > rootfs.img (9,945,232 bytes) is there a problem with unsquashfs or mksquashfs? because when I used a hex editor software, I noticed some entries are different although I have not changed anything. **EDIT** someone suggested that the header.img file could contain the information about the rootfs.img file and any change could cause an upgrade failed issue unless I match it with the info inside the header.img , how would I edit the header.img file, I can't unsquash it, only view it as hex and all the text inside is rubbish.