The factors in the data aren't just your voice but recording environment and microphone, and a recognition "engine" is really a combination of model architectur

The factors in the data aren't just your voice but recording environment and microphone, and a recognition "engine" is really a combination of model architecture (sometimes separate acoustic and language models), training data, hyperparameters, and preprocessing. So the reason you don't see these is that there is an enormous hypothesis space, with only small regions that are performant.