I've always considered car rental prices similar to airline fares... I guess if they just happen to have some kind of promotion, the promotional (non pre-pay) fare can be lower than the regular pre-pay one.

Of course there is a potential currency exchange rate risk. Though I'm not really sure about that, I've booked a 'pay at rental station' car in SFO which was quoted in GBP (without the indication that it was only an estimate and would be charged in US$).