I recently noticed that the cost per conversion numbers shown in AdWords (1-per-click) were wrong. They were under-reported by as much at 15%. Simple long division (Cost divided by one-per-click conversions) was not what was showing up in adwords.
I called and IMd support and they have no idea what would cause this. Of course, being Google, they would never EVER admit a mistake so they just say they will get back to me.
Basically, this leaves two options 1) Google screwed up math or 2) They have up and decided to count something else as a conversion (I thought it might be phone calls, but that doesn't ad up either.)
Has anyone seen anything like this?