[ Habit is the likeliest and most plausible answer. Bruce has lived over forty times longer than Vision has, for one, that's a lot of time to grow and get used to one's own body and movements. When Bruce was a year old he also lacked severely in grace, as well as balance, coordination, what have you. One grows and learns, though.

Bruce smiles when Vision mentions his job assignment, but he doesn't find it especially amusing. Instead, he just shrugs and offers simply. ]

You're not a robot. [ Honestly, that's far too simple a way for Vision to describe himself. ] Somehow I have no doubt that you'll find your own way to do this job better than a lot of other people on board could.