How things work in general are that there two sets of "weights" per item type, plus a weight for no item, used for choosing held items and as part of choosing random drops, e.g.:
Hyper Goomba
Nothing - 200 H, 300 D
Life Shroom - 5 H, 5 D
Tasty Tonic - 0 H, 5 D
Ice Storm - 5, 5
Boo's Sheet - 20...