it can be a delay in the reporting... just wait some time and you should see an update for impressions number.
I have seen it in my account. It may happen when u have problem with channels. If you are using channels and you put wrong channel or move account but keep the channel same or someone else put your channel then this can happen.
The stats just updated for today (3AM EST) and I am looking at 2 page impressions and 3 clicks on a channel (there are none for the other channels and I have no pages that are not channeled) ... kind of scary but I hope it's just something wrong with G reporting them or maybe someone clicked on an ad on a page cached by MSN ... pretty fat clicks too... hate it when this happens
It happend to my account frequently... i think 11:59 pm the page loads... impressions made... after some min... 12:01 am... clicks made