Any times I have noticed price discrepancies from travel sites, nationals website sometimes carries the cheaper rate vs priceline, orbitz, expedia ect Also one way rentals out of Florida seem to be very cheap!
1. Maybe National has your discount number (such as EC) programmed in while those others don't! 2. Surpising this time of year, but most car-rental co.s offer bargains on one-ways out of FL in Spring, when fewer cars are needed in FL & more up North!
I think the larger travel sites have limited visibility into National's rates, unlike for Hertz and the other guys where the rates are very similar.
national will most likely beat out third party site. Unless there is rare occasion of them having inside promotion.