Monster Bookworms

x2 drop does help but think of it this way

Lets say you have a random number generator and it gives you numbers from 1~10,000

A x2 drop card works (Or is supposed to work) so that it runs through the probability twice.

So the Prob(1) = 1/10,000. Because of the card, it'd be 2/10,000

While it is an increase, relatively it's still very low.

2x cards works so it runs through the probability once more if the item hasn't been dropped yet:

P(1) = 1/10,000
P(2) = P(1) * P(not P(1))
P(2) = 1/10,000 * 9,999/10,000 = 9,999/100,000,000 ≈ 1/10,000
P(at all) ≈ 2 * P(1)
Here droprate roughly doubles.

Compare with a 60%-droprate item:
P(1) = 60/100
P(2) = 60/100 * P(not P(1))
P(2) = 60/100 * 40/100 = 24/100
P(at all) = P(1) + P(2) = 60/100 + 24/100 = 84/100

(Oh well)

For 2x drops however, they activate something which doubles the rate.
P(at all) is thus 2 * P(1) as long P(1) is lower than or equal to 50/100. Otherwise, P(at all) = 1 (always drop)