I would think like your thinking in regards to a space marine who has centuries to develop theses skills. But I am thinking of humans. Who have decades, probably less to do so. And have to have a more limited but specialized skill set. 

My idea being ya the guy who has been piloting for 20 years may be a fantastic pilot. But that does not mean he can or has had the time to drive many tanks around. because most of his time is spent being a pilot, where he shows a particular aptitude and the military won't risk in a thing he may not be competent with, or find any reason to pull him away from that field of specialty he shows so much prowess in.