The detailed "why" would require inside information as to the workings of
Access, but it's routine behavior. Access must keep track of... something...
user security, query stats, who knows what!

If you need to keep track of when data in your tables is updated, Access won't
help you; it neither tracks nor exposes that information. You'll need to track
it yourself, not an easy task unfortunately!

            John W. Vinson [MVP]

I suspect Access is either changing the query statistics or possibly
changing the form filter.  I've noticed that you can use an app for a
few minutes and exit without changing the date/time.  But any more
specific I haven't bothered to do any research.

Tony

It's just the way it is. The Access development team decided it was
the right thing to do.
Of course if you do a binary comparison of the file before and after a
simple Open (perhaps using fc.exe in a command window), you'll notice
that it has in fact changed. Perhaps some internal flags, perhaps a
new compilation state, we are not privy to those finer points.

So I would say if the Archive bit is set, better back it up.

-Tom.