I think this means that 1 in 5 pages hosting a third party ad network has no relevant content for a legitimate web user, so visits to that site must be by bots.

I think this means that 1 in 5 pages hosting a third party ad network has no relevant content for a legitimate web user, so visits to that site must be by bots. Not saying that is what happens, just a way of interpreting it in a way that is not impossible...