75% seems pretty high, but I think it's normal to have more subvols than are needed. In my class, I know there were quite a few subvols who were surfaced. I think they spoon feed us that there's a shortage of subvols, and pretty it up to try and convince everyone to volunteer as a just-in-case kind of thing. It's kind of a one way street, and it's a lot easier to surface a subvol than to force someone to go subs. And at 75%, there's going to be a large fraction of people getting surfaced. The best advice I can give to avoid having it happening to you is to finish at the top of your class, and qualify early at prototype. Those are the ones who tend to get first dibs at what they want.