We've got an alternative method of counting unique impressions - we count as unique only globally unique impressions. That's why our numbers are different from what you see in Google analytics or on other ad networks.
Counting globally unique impressions means that one IP can be counted as unique only once a day within the whole A-ads network. So a visitor produces a unique impression when he first sees any of our ads on any of our publishing sites and he doesn't produce another unique impression this day, no matter what sites he visits and what ads he sees.
So unique impressions numbers you see on our network will normally be less than those you see in other places. But don't worry - it doesn't affect your earnings as the counting method is the same for all our publishers.