I think the amount time you spend in the saddle on a single horse makes all the difference. I think *most* saddles don't fit well and the only reason no one is the wiser is because the rides are short and infrequent.
For example, when you ride 4-6 hour trail rides with only one horse, 5 days a week, you are going to see white hairs unless you have a REALLY good fitting saddle.
I have gone through saddle fitting HELL and finally got a saddle (wade tree) that didn't give my Mustang white hairs from a long summer of riding.
I used to think it was just that I had a weird shaped horse, or I was heavy, etc. But my neighbor whom I ride with weighs a lot less than me, has registered QH's and top-of-the-line saddles and she has white hair issues on her horse's backs worse than I do now. And she had two horses she could ride while I was only riding one all last summer.
She started with Cactus saddles, then had a custom saddle maker make two saddles for her (one for each trail horse), which she rode with all summer, and they gave her horse's white hairs worse than the Cactus. I think the problem was the saddle maker was out-of-state. Just how can you build a saddle for a horse out-of-state, anyway?
Now she is working with another custom saddle maker to get 2 saddles made for her big, beautiful, registered QH's. If you can't fit a QH in a western saddle, what can you fit, lol?
So I dunno, I think you REALLY have to put in a lot of hours in a saddle to know how well it fits, and then the white hairs may not show up until the seasonal coat change.
What is nice, that I have found, if you start getting white hairs and you switch saddles as soon as you notice them coming in, those hairs will usually shed out and grow back in dark the next coat change.
And even if a trainer rides a horse daily, he is probably not putting in 5 hours on a single horse, so his saddle may not be a perfect fit either.