"You lvl positions with films sold." - this part is incorrect. You gain a fixed amount of exp for selected position that depends on room, not the amount of films sold. Amount of films sold increases with higher Stamina but only affects amount of experience that Industry where you sell those films gain. Girl that gets a 10h movie done in first room will always get 500 exp to her selected position.
Just look at actual numbers of experience required for each position and check how much you gain with each movie. It will be the same every time.But you also increase films sold in the same room with the same girl if you shoot another movie. Just look at the number of films sold when you collect the movie and then at the number if you send the same girl again. It is higher. Not high enough to compensate for the increase in position lvl cost, but it is higher.