Well to be pedantic, because that's what you're doing, he didn't drift to the right on the goal. Cavani was on the right for the goal. Rashford was central.
And playing him on the right generally doesn't work. Playing him on the left does. It's not just the Southampton game where this is true, but his whole career. He interchanges positions when he plays on the right, too, but it doesn't do anything to improve him out there. It doesn't suit his strengths. Our forward players generally swap positions during the match. But if you think that where a player predominately plays during the WHOLE match and where he starts has no bearing on how he performs and the positions he takes up, then I don't know what to say to you, to be honest.
You seem to be implying that it doesn't matter where players start, just throw them on the pitch. Their positions don't matter? And one more time just to get this through, this isn't just the Southhampton game I'm basing it on. I happen to be commentating on the Southampton game because, well, it's the most recent one!
Would never have thought that saying that Rashfords best position is on the left (or maybe centrally) would be so controversial to you.