Edit: this is player Mike speaking.
For me personally, the distinction lies in the name really. A Mastery being something that you spend those 10,000 hours in and it becomes part of your identity. It sure doesn’t take that long in game but that’s how I view the choice. It’s your character cementing their identity in the world. They transition from being a simple Rogue who is still finding their way in the world and experimenting into a Bladedancer, an elegant weapon who wields blades with the precision of a master.
Edit: developer Mike here now
From a gameplay standpoint alone I can totally see how it feels arbitrary. It is something we have debated many times and I’m sure it will be debated again. The feedback does not fall on deaf ears. For now however, we do not intend to change this. Who knows though, maybe we end up with a couple new masteries for each class and get to a point where having so many classes just becomes unwieldy. The game will continue to evolve and grow.