Premium can be cheaper than standard for the same reason that occasionally first class seats are less expensive than the least expensive coach seat on an airplane. When the pricing specialist (or an automated computer program) adjusts the prices on the coach seats (or the standard cars) due to supply and demand, but does not adjust the higher class of seat or car. Usually this is due to some sort of pricing mistake, although not always.

Also, in the case of cars, if 90% of standard cars are reserved already, but 80% of premium cars are not reserved already, the price of standard cars would be increased in order to push rental prospects into the next car class so as to insure that class of cars also gets to a higher level of reservations too.

Finally, right now, smaller cars are almost at a premium, because people want to save on gas.