You're getting an awful lot of junk in your deck so you discard a semi-useful card (Copper) to draw 3 cards of which 2 are probably junk. The simulator sure doesn't like the Stables in the Mountebank deck. It even suggests it's worse than Silver! Laboratory on the other hand it likes (compare these hands : "Stables, Stables, Curse, Estate, Mountebank" and "Lab, Lab, Curse, Estate, Mountebank")