Well I think it comes from the fact that half of the session will consist of a blowjob, and the other half vaginal sex... in terms of time.
Some girls will have a BJ rate, a vaginal sex rate (usually 50-100% more than a BJ), but will just add the cost of the two to come up with a half and half rate, but still give you the same amount of "time"... so be careful of that.