My page impression is always showing a value higher than the total impressions of all my sites .. Anyone knows what i need to do to find out were the excess impressions are coming from?
Most likely you didn't channel some pages, thus they aren't showing up in your sites listing. Go back and try to think about old pages you haven't touched for a while (thus you didn't properly channeled), and that's probably them.
How big is your site? And how many sites do you have? You can't possibly be 100% certain you've channeled them all, can you?