replay doesn't catch up with receive on standby

This is on 9.0.3: I've got two dbs running as standby to a main db. They start up fine and seem to think they're all
caughtup (by /var/log logs), but 

SELECT pg_last_xlog_receive_location() AS receive, pg_last_xlog_replay_location() AS replay;

reports replay behind receive and it doesn't change. This is on both dbs.

Notably the main db isn't (wasn't) doing anything, so no new commits were causing things to move forward. I did a write
toit and both slaves moved both their recieved and replay serial numbers up. 

Is there a valid situation where an idle master/standby setup could remain with replay behind received indefinitely?
(Mynagios monitor isn't very happy with that (at present)) and before changing that I'd like to understand better
what'sgoing on.)