Most of the time, pros simply get sent product and have to get their boots fit and skis mounted on their own. And when that happens, lots of mistakes can happen.
In my experience, pros often get the wrong size boot (when I fit Tom Wallisch he had a 3cm shell fit and I had to swap out his boot with a different size that I had in stock) or at the very least they need to get boot work done because they are just like anyone else and have small weirdnesses taken care of.
Very rarely do you get an athlete who just doesn't fit in anything a brand has to offer. This is mainly due to the points Tom has already said but also most pros tests gear before they sign a contract. This is going to be their job for the next X amount of years, so if they take it seriously they make sure they are going to be happy wearing the gear all day long.