I bought a 1940s vintage roping saddle and by actual measurement the right stirrup was an inch higher than the left even though they were laced in the same set of holes. I had to punch a new set of holes to get them even.
OP, I know exactly what you mean, I've noticed that as well. I don't have a great explaination for it, while the leathers stretching does make sense, I just bought a brand new saddle that I've only ridden in once so far, and on that first ride felt that one stirrup was shorter than the other. The stirrups did tend to feel more even as I rode though, like they usually do in every saddle I ride in. So my theory is that if you mount on the left side your left leg will always feel longer because of the weight you're putting on it, while your right leg just comes to rest in the stirrup with no weight. Basically it's psychological, not really the case.