I realize I'm a little late on this, but . . . If you've purchased the ticket, the price will be locked in and will not change. They are just saying that it could change between the time you request the reservation and the ticket purchase is actually completed. In most circumstances, this occurs instantly, so there isn't a problem. If there is a credit card problem though, for example, the ticket would not be purchased and when you go back to fix the problem, the fare may have changed.

FWIW, I run the travel site for PriceGrabber.com (also a metasearch site), and Airfare.com is one of our partners. We have yet to receive any sort of complaints about them to date, so hopefully these are just standard disclaimers and not often seen in practice.