I tried the exact same example as used in the hint, having applied the system update shortly after it came out.

Looking at modified time (with Mail highlighted in the Finder), the back arrow stopped several times before getting to the file that had a noticeable timestamp change, although it was skipping many backups each time before stopping.

Am I doing it wrong or is the backup stopping on something else? Except for evil programs that force themselves into /Applications, I don't normally put anything there besides Apple supplied files. I didn't look deeply into it, but I was guessing that it was stopping on any change in the current directory.

I'm not sure, but my guess is that you have a folder hilited, not a file. When you click the "search back in time" arrow, it is perhaps searching for the last change to any file in the folder. Just a guess.