When the aggregate plugin was used for a feed and this is removed (or the same feed name given a different rss feed), the old entries don't automatically vanish.
I think that if it was just removed, they are never GC'd, because the expiry code works on the basis of existing feeds. And if it was replaced, old items won't go away until expirecount or expireage is met.
To fix it probably needs an explicit check for items aggregated by feeds that no longer provide them, Catching old items for feeds that were changed to a different url may be harder yet. --Joey