I am not sure this is normal. I have two sites. Site A, 1 year old. Site B, 4 months old. Both have two blocks of adsense there. Site A: page impression: 2000/day, ad unit impression, 16,000/day Site B: page impression: 15,000/day, ad unit impression, 16,000/day They have so much difference in page impression, but almost has the same ad unit impression. What can explain this?
Basicly if you have more than one ad unit per page, your page impressions will be less than your ad unit impressions.