Hard disk aftermath: Same files have different checksums

I just recently returned a hard disk that I had been having latent problems with for quite some time and was getting ready to free the space needed for backup of two rather large disk image files only to notice an oddity that needed further investigation. Whenever I have a problem of this sort, the first thing I do is compare the MD5 sums between each of the files that are supposed to be identical. Something interesting happened, however, and that was that although each disk image was from the same backup (one original and two backups for the first disk, and one original plus three backups for the second disk), the first disk image checksum differed from one of the backups, although it agreed with the other backup. For the second disk, a total of three distinct checksums (!) were found for what was supposed to be the same file. The problem is further compounded by the fact that I do not remember what order or time I made the backups in so it is not possible for me to make a definitive conclusion for any disk image that the images with the matching checksums are in fact the original files despite sharing the same checksum. When I ran Windows File Compare on the files, all it would tell me was that some files were longer than others, although in Properties the number of bytes were identical in each case it said this. I have restored each disk image mentioned successfully. Each disk boots as expected; however, all of the searches I did on each disk revealed no discernible difference except for one that seemed to be running faster than the others, although I regarded the difference as negligible (each of the images was burned to the same hard disk and examined separately; therefore, hardware is ruled out in accounting for the difference in speed). How can I figure out which disk images are the originals? I have tried just about everything I can think of, and yet I do not even have a notion of which ones I suspect are corrupt. As you can imagine, the hard disk image files are quite large, and the inconvenience of having to store them on several disks I need to use has become a situation that must now be definitively resolved because of pressing considerations.

Not sure if this will help or not, but the disk image files are Acronis True Image Backup (.tib) files.

Maybe there is a date/time stamp in the files that would cause a different checksum?