There are a few possible causes of that:

1. You've done an upgrade previously and perhaps had your WinMerge or other comparison program set to ignore the version-id lines ... and thus when you merged the new changes into your files, the version info was missed.
(This is the most common).

2. Perhaps you installed some patched files (perhaps the PayPal patch) and that brought the "content" up to date, but when the new version ZIP file was prepared, the code-mgmt tools updated those patched files with a new revision. However, since you already had the updated "code", the only difference you see is the date/time/rev stamp.

3. Perhaps the developers touched a file and maybe decided to "back out" of a change before releasing the new code. The new version stamp info would still appear in the file, even if nothing else was changed in the end.