I think it's the type of sales that counts too.. for example:
A halo game will always be frontloaded and sell 4-5 millions first week, same as CoD. That first 5 million doesnt come from the quality of the title but from hype. However legs for example develop when a game gets a good word of mouth equating to quality at least to a respective group of people.
Wii fit for example is a quality exercise game which is why it had ginormous legs. also the legs of CoD: MW is another example of a game that legged out on contrary to GTA4 which was much more front-loaded(and that's not considering how CoD had sequels to hinder MW's sales). And before you attack, I AM NOT SAYING GTA4 WAS A BAD GAME, just that the reception of Call of duty was higher this gen.
So the question really cant be answered the way you panned it out to be
