When you see an item spawn, the weight of that item is automatically decremented inside of the respective item pool.

Item weights were adjusted more extensively in Repentance.Plan C has a weight of 0.2 in the Curse Room, Devil Beggar, Devil Room, Greed Mode Shop, Red Chest and Shop pools, making it 80% less likely to encounter.Mega Blast has a weight of 0.2 in the Devil Room and Greed Mode Devil Room pools, making it 80% less likely to encounter.Delirious has a weight of 0.4 in Angel Room pool, making it 60% less likely to encounter.Bomber Boy has a weight of 0.2 in the Bomb Bum pool, making it 80% less likely to encounter.Only a few items in the game do not have a weight of 1:.((1 * 198) + (2 * 0.5) = 199) This means items with a weight of 1 have a 1 in 199 (0.5%) chance to spawn, while an item with a weight of 0.5 has a 1 in 398 (0.25%) chance to spawn. In a pool of 200 items that has only 2 items with a weight of 0.5, that would result in the total weight being 199. However, if an item has a weight of 0.5, that means that item is 50% as likely to spawn compared to an item that has a weight of 1.For example, if there are 200 items in a given item pool, and each item has a weight of 1, that means there is a 1 in 200 (0.5%) chance to get any particular item from that pool.Nearly every item in the game has a weight of 1. The weight represents how likely it is for the game to spawn the item. Each item that is in an item pool has a weight associated with it.

Binding of isaac rebirth items