My understanding...
In order for your UV to do anything to your water... it has to travel thru the UV unit, then at a slow enough rate (depending on the unit's size) to actually have any effect on the various organisms carried in that water.
I have a TON of pods in my tank... mostly on the live rock, and down in my Fuge. I'm sure "some" pods do manage to actually run thru my UV unit... but the amount is so little, it doesn't effect my pod population.