Now I ask you.
After finding the 1st ab, I calculate to find the 2nd ab.
I calculate to find the 3rd ab based on the 1st and 2nd ab.
After finding the 1st, 2nd and 3rd ab, I find the 4th ab.
Now, are you saying that these are not probabilities but predictions?
Are you saying that probability never changes?
Or are you saying that there is an independent probability but that there will be no conditional probability?
Once you find "ab", then the probability is no longer a probability, it's a certainty. It just proves, in long-term average, that the probability / prediction was correct. So, once you find an "ab", that is great and all, but it doesn't affect at all the chances of the interval that follows. You're still at a 36% that no "ab" will be found in the next 256 samples. And still at the same probability of 1/256 that it will be found in the next sample.
Results are only useful to prove predictions, not the other way around. Maybe you found some way to use the conditional probability, as you call it, into the mix, and maybe that helps to find better prefixes, I'm not saying it doesn't. But that doesn't change at all the chances that you can always skip the key you're actually looking for - it's the same.