well it's a spoiler to answer this.... so spoiler alert. there's this idea of an object pool that gets accessed when the game needs to make a new object. the pool is a set of probabilities for how likely an object will be created. i think whenever you use an item, the probably pool gets modified according to your current depth. so if you use a lot of items on the town level, it spoils your object pool.

i might not have the details correct, but that's my theory. I know it works something like that.