I'm sure that's right. There are more eyeballs doing searches on Google, and there is an auction between advertisers pay to put their ads in front of users. More advertisers chasing users probably gives them more revenue per search.

Shouldn't there be some sort of parity?

A user is a user is a user. Unless google users are that much more likely to buy something after clicking my ad, why would you pay more per individual view? Sure the absolute numbers should go up with more views, but it doesn't seem like money well spent to pay more for similar adspace.

That would assume that people using Adwords compare costs with the other ad networks before deciding to advertise. I would guess this is most often not the case.

Actually, for some industries, Google traffic converts much better than Bing/Yahoo traffic. No 2 traffic sources are exactly alike.

For years Yahoo! converted 30% better than AdWords on the same keywords for several of my active CPC clients.