Impressions can be miscounted if they are cached by the players PC’s.
Most IE users run a local cache on their PC to speed up browsing and if an image location has been seen before then the browser may use a local copy rather than pull it from the server.
This would mean a highly exposed property (of the exact banner) could be cached frequently by returning users … especially if the banners are displayed at the top of EVERY forum page (for instance).
Although why one company would be noticably different from another is debateable – if they have exactly the same exposure.
(just an idea).
:huh2: