I'm not sure about what you are seeing but here is one
possibility.

   Normally there is a "spam delay" of about 10 minutes if the
From: address does not match a subscriber.  I think the idea
is to be able to gather information on mail items going to more
than one list or server and trigger a response within the 10
minute "cooling off period".  You can set this for a list
with the Loopcheck= Spam-Delay(mm) where mm is the number of
minutes.  It can be set to 0 for special cases but then you
defeat the idea of being able to gather information and react
to incoming spam notices.

   Marty