Google now makes it possible to assign multiple custom channels to a single ad unit. Google AdSense blog announced that you can now track up to five different channels on a single ad unit.
"What's the benefit of tracking with more than one custom channel? Well, multiple channels can be very useful when you want to track one ad unit across several different metrics simultaneously. For example, let's say you run a sports website and you've placed a leaderboard at the top and bottom of every page. To track the performance of the ad placement, you've created two custom channels — 'TopLeaderboard' and 'BottomLeaderboard' — and regenerated your ad code appropriately.
But what if you also want to compare your football pages and your baseball pages at the same time? With multiple custom channels, this isn't a problem. Just create two new custom channels called 'FootballPages' and 'BaseballPages', and add them to the appropriate ad units. Now your leaderboards will each be tagged with two custom channels that let you know which position they're in (top or bottom), and the type of page on which they appear (football or baseball)."
Ad units tagged with multiple custom channels will log impressions/clicks in each channel. Thus, it will show higher impressions on channel reports than the aggregate reports.