I dunno, it seems to me that each size of speaker playes a certain frequency 'optimally' (as in, you don't see very many 12 inch tweeters)... sure they can engineer different sized speakers to play frequencies that may otherwize not be best reproduced by that particular size. The 6 1/2 inch divers in my M80's are a great example. In all my car audio days I never, and I mean NEVER heard speakers that small produce such impressive bass. So my guess
would be that your AVR is trying to send the bass signal to your subs that best suits the size of your sub. But with todays subwoofers, that just might be a non-issue.
Either way, I'm sure JohnK will be along shortly to tear apart my theories and set you straight!