Thanks for the reply Derek.  I also thought it looked like a bug, and
your explanation makes sense.  However, Markus Mottl pointed me to the
following  issue in the bug tracker

  http://caml.inria.fr/mantis/view.php?id=3476

The problem there looks very similar, and would seem to indicate that
the situation is not viewed as a bug (in the sense that it won't be
fixed).