[currymj]

One problem with second price auctions is their lack of what is known as "credibility". Google has both the incentive and opportunity to lie to the highest bidder about what the second-highest bid was, so they can charge more.

They never literally did this, but ended up doing some convoluted scheme involving subsidies and rebates that, according to a recent lawsuit, amounted to something equivalent.

First price auctions don't have that problem -- you know exactly what you should pay if you win.