This was caused by one of two things. Either: 

Bob’s Buddy couldn’t run enough simulations.
​Your computer wasn’t able to complete enough simulations to accurately predict how likely you were to win, tie, or lose.

Your opponent had Secrets
​Secrets controlled by your opponent are hidden information, so we can’t take them into account within the simulation. Eventually we will take add support to the secrets that are revealed over the course of combat.