Each hash is essentially an observation of a uniformly distributed variable (hopefully). When you flip a coin, it's heads with probability 1/2 and tails with probability 1/2. The fact that you flipped heads on the first go doesn't change the probabilities. In the same way, the fact that your hash didn't match the difficult on the first go, doesn't change the probability it will match the difficulty on future hashes. So each hash is independent of the attempts you've already made, so there's the same probability of a match at any time step.