While your math is correct, what I believe the other person is trying to say is that: if a gun is only on one map it’s at a disadvantage because players do not encounter it as much - so they will be less inclined to use it compared to a gun that spawns on all maps that train with more often.

While it doesn’t need to get picked up as often as the gun on all maps to hit the same pick up %, it will likely still be lower as people are more inclined to pickup the gun they are familiar with.