Why does P2Pool show up on BitcoinWatch.com if it's P2P? I was wondering if P2Pool is decentralized why does it show up on the BitcoinWatch pool distribution graph?

I thought it would show up under "Unknown" but I guess this is only solo miners? Or some dark pool?

And "Other Known" I assume is a selection of small pools whose hash rate is too small to have a visible slice on the graph?

EDIT: It's currently not visible on the graph on BitcoinWatch but I'm certain I saw it on there not long ago so I will upload an image when I see it again.
All miners in P2Pool mine "solo" but distributing the reward between all its peers, hence having both advantages of solo mining and pooled mining. The reward distribution is build into the block itself, you can't mine such a block to change it later to give yourself all the reward, nor will the P2Pool accept shares that don't contain reward redistribution.

If it doesn't show as a pool it's encased into "other known".