This is a fairly common tactic when you have intermediate in-game currencies that you actually use to buy stuff in the game. For example, you can only use gems to buy stuff in-game, and most items in-game are worth a multiple of 200 gems. But when you buy the gems with cash, they're somehow only offered in multiples of 500 that are also not multiples of 200. You end up buying a few extra that maybe you didn't want, and they can later hook you in with the 'well, you already have some in your wallet' approach.
This is a fairly common tactic when you have intermediate in-game currencies that you actually use to buy stuff in the game. For example, you can only use gems to buy stuff in-game, and most items in-game are worth a multiple of 200 gems. But when you buy the gems with cash, they're somehow only offered in multiples of 500 that are also not multiples of 200. You end up buying a few extra that maybe you didn't want, and they can later hook you in with the 'well, you already have some in your wallet' approach.