[AccessD] Question - subform filter

Sometimes, it just doesn't make sense - to me anyway.

I have two databases, virtually identical. One was working, the other was
producing strange results.
I have a form with a subform.  The subform dynamically changes to either
have multiple subforms or a single subform.
When the subform changes to have multiple subforms, in one file it would
work - the other would not.  The contents of the subforms would not change.
After much work... comparing code, properties, data (data is identical in
both), importing and exporting objects. I finally found the problem.

Apparently, if I want to ensure the subforms change dynamically, with
different records, the Filter must be set and cannot be left blank.  Even
though the filter changes for each record the data of the subform does not
change.

Why is that?

Thanks
Jim