From their FAQ:
I guess they just exchange the EUR rate to USD to make it easier for everyone.
Quote
With which currencies are Bitcoins traded?
Momentarily, Bitcoins can be traded for Euros (EUR). We will try to push the trade forward with other currencies as quickly as possible.
Momentarily, Bitcoins can be traded for Euros (EUR). We will try to push the trade forward with other currencies as quickly as possible.
I guess they just exchange the EUR rate to USD to make it easier for everyone.

Yes, read my first post, I read their FAQ too; however the USD price is different (and sometimes even very low compared to the EUR price and other markets price) so it can't be that is just the eur rate converted to usd!