[chewxy]

Simple. Pages with long content under the fold. Most of the ad tags are called with page load, and there will be many situations where the ads will be served but not seen. The situation cannot be helped.

In case you're wondering about asynchronous ad-loading, i.e only load an ad when the tag scrolls into the viewport, no that is not a viable option because the world out there is crazily varied, and not everyone uses the latest nightly build of Chromium. There are a lot of bad javascript engines out there, so I wouldn't jump to assume that async loading of ads would work across the board. Trust me I've worked on this problem before.

As for why different pricing would work: what I had meant was billing post-facto.

So for example, you may have an IO saying you're buying 300k imps per day at $1 CPM viewed and $0.1 unviewed. At the end of the day you may have 150k viewed and 150k unviewed. You only pay $165 instead of $300

EDIT: I have bad arithmetics for an asian person