Welcome to FlyerTalk, alocksley.

Multiple possible reasons for the difference:

1. It probably costs a lot more to guarantee an exact car than to guarantee just a car category (which may even be "stretched" in case they upgrade you to a different car category). So IMHO you're comparing apples to oranges? Does Hertz not have a category option? If so, compare the category option at Hertz to the category option at Avis, not just the exact-car option at Hertz to the category option at Avis.

2. The cost of a one-way rental is determined by how much they need the car where you're taking it from and how much (or how little) they need the car where you're dropping it off. When you rent just a category, they can give you whichever car in that category they think they can live without at LAX or that SFO could use. When you rent a specific car, especially if that specific car is way more popular around sunny LAX than around foggy SFO, they essentially are charging you for dragging that car off to somewhere where there's much less demand for it.

But even if you were comparing category rental to category rental, one company might have more need of cars at SFO than the other, and that company might discount one-ways to SFO for that reason.