Even if the opponent is better, the inherent randomness of Dominion often makes the mirror the strategy with the best expected value. Deviating from the mirror requires a higher reward, even if there is a much higher risk, than the mirror can obtain for it to possibly be worth it.
Your turn by turn decisions won't be exactly the same, and the timing of particular gains or the priorities you choose can be crucial. This is where Dominion really gets interesting, not as much in "x decided engine and y decided big money let's see what's better" high level stuff.