File Last Changed

I have been trying to analyze the *File objects in a library to determine if I really, really need to keep them. One of the tools I'm using on the questionable ones (i.e., no one claims responsibility and I can't find anything that references them) is DSPOBJD.

The weird part (to me) is that a lot of them have a "Last Changed Date" of 26 May 2007 (a Saturday) at about 20:00. I thought that might be because of a PTF (Cume or Group) because I would have done that on a Saturday; in fact, my time log indicates that I did work that day and, other than a natural disaster, applying PTF's is about the only reason that I would have worked on a Saturday. However, I know applied a CUME and Groups to that system on 20 October 2007 (a Saturday, naturally); so that idea doesn't seem to hold water.

And most of those files exhibit a "Last Used Date" earlier (sometimes by a couple of years) than the "Last Changed Date". But, if I (or anyone) actually changed the file in any way, then the "Last Used Date" should get updated, too. Right? Well, that's what my tests (simple DFU on a file) showed, anyway. A simple query against a file affects the "Last Used" but, naturally, not the "Last Changed" date.

I have not been able to find anything in the Info Center (V5R4) or the Archives that would explain why the "Last Changed Date" would be later than the "Last Used Date." Anyone on the List know (or simply want to hazard a SWAG)?