You must have gotten lucky. That happens occasionally try farming them today and see if you have the same success. I too have had similar success in the past. I've had two Exotics drop from 1 Ultra boss before. But just because it worked when you used it last night using 5 three of coins to get 2 Exotics also May mean that next time it's going to take 25 for you to get one so unless you've been farming or testing it you can't say that it works only using 5 three of coins