Don't worry about overthinking.
When I hadn't seen the Mana Window yet, I also thought about, how hits by a resistance roll from the barrier, will diminish my spells hits and what that meant.
Then I saw in the Mana Window spell and that they did exactly that.
I think (overthinking again) that when I have 5 hits with Mana Window and for example a Ward steals 2 with its roll, than I just cannot see as good as I could at the places, where there was no barrier.
Because the general description of a passive detection spell (and Clairvoyance and Mana Window are passive) says (page 206 SR4):
QUOTE
The Spellcasting Test serves as a magical Perception test, against an appropriate threshold determined by the gamemaster. Other effects are noted in the spell descriptions. The Detection Spell Results table provides guidelines for how thoroughly the sense works, based on net hits scored
So your net hits in Mana Window are your Perception (that might be good or bad, depending how good your Perception skill is).
The barrier's resistance roll hits will not put the target out of range, but they will "dull" the picture. The range of Detection spells is not dependent on hits, but always Force * Magic.
What you see will be less clear if you loose hits to the barrier. That's how I would interpret the quote above.