I'm wondering if they aren't meant to increase your baseline chance, but instead implement "bad luck protection" like Legendaries on WoW; every time you DONT get an Exotic with the 3oC active, your chance goes up slightly until you get one. Might explain why people get nothing and then all of a sudden they do once the coin expires, and why it makes the coin feel worthless since you spend most of the buff's duration building your drop chance. Totally random guess, though.