Reversibility of aacgain Options

[BrownNote80]

In the past I have used mp3gain and aacgain on my library. I recently "undid" all of my files, in an attempt to return to original gain levels. It appears to have worked fine on all files, both mp3 and aac, and I have not observed any issues. However, I am very curious about how "different" the aac files are now compared to the original files. According to a description at the aacgain site (apologies for all-caps),
"AACGAIN IS NOT COMPLETELY REVERSABLE. THE UNDO OPTION IS INTENDED TO RESTORE A MUSIC FILE TO BE FUNCTIONALLY EQUIVALENT TO ITS ORIGINAL STATE, BUT IT WILL NOT BE BIT-FOR-BIT IDENTICAL TO THE ORIGINAL FILE."

My question is: what does the above mean? Would the "bit-for-bit differences" affect sound quality at all, or would it only be from a playability perspective (i.e. corrupted or unplayable files)? I'd just like to know what changes have been made. I do have backup files of the original aacs (as well as the cds), but I'd prefer to use the current "ungained" versions rather than re-import my backups if possible. Many thanks.