steve40 - 12:24 pm on Mar 5, 2007 (gmt 0)

Well here is another possibility

Advertiser sells and is targetting

Green Widgets

Green Widgets CPC 50cents ( good Conversion )

Widgets CPC 5cents ( Poor Conversion )

Scenario
Publisher Site increases traffic through being higher in serps for widgets or lower for green widgets

So G shows Widgets add exactly the same add but now only paying 5 cents

To the publisher all looks exactly the same but due to advertiser campaign overall quality score same adds but 10$ of CPC

Now this may be the only advertiser doing this on green widgets and the others may well be paying more so new range could be from 10% drop to 60% drop and all would look the same as before to the publisher

PS one other thing this could also then effect may be quality score of publishers site ( I.E. less conversions )

Just my ramblings and difficult to fully explain

steve