There's no catch. That's because a CPM ad is also running on Google Adsense. They pay per 1000 impressions. See more details here: https://www.google.com/adsense/support/bin/answer.py?answer=18196
I see that from time to time inside my account, too, mostly coming from digitalpoint. Not paying any attention to that though
As kukote says, you're being paid for impressions, and if I had a nickel for every time this was asked on here I'd be retired