Because of the high number of 513 games, neither an arithmetic nor a geometric series with ratio 2 seems realistic. An arithmetic series would mean that the bestselling game was only sold 1.25 times as often as the 100th bestselling game. A geometric series with factor 1/2 would mean that the 5 bestselling games account for over 96% of the total number of games sold. I'd suggest to try either a geometric series with a ratio close to 1 (some quick calculations suggest that the ratio should be a bit less than 1.025) or some more complicated approximation. I doubt that either of the two simplest models gives a very good approximation.
Oh yes I totaly agree, I only started to speculate out of frustration (I want to knoooow !). Given the almost non existant information we have, my hypothesis are reaaaaally weak for starters.
In all cases, we shall agree that such an approximation is not fitted for the top games sold and the least ones (statistical rule if you see what I mean).