You've almost got it - it probably IS a "longest-retention issue".

Andy Raibeck discussed this at length in Oct 2001 (if you want to go back
and search www.adsm.org, search on :
Raibeck Strange Behaviour

Andy said:
...."If two or more management classes have the largest RETONLY setting,
then it is
undocumented as to which class will be selected."

"Undocumented" means, I think, random results.  TSM is not smart enough to
prefer the DEFAULT, if the RETONLY settings are the same.

Change your DEFAULT class to be 1 day longer than the others, and I bet the
problem goes away.