I looked at the rules, its not the highest scoring sub, but the one with the highest priority. So it is important to put your subs in a particular order. But you are correct on the position requirements.
For example, I had 2 midfielders not play at all (Nocerino and Alashe). My bench order was Johansen, Amirikwa, Polster, Vincent, Sjoberg and I played a 3-4-3.
Nocerino and Alashe got 0 minutes in the midfield. Since I used all forward and GK spots I could, Polster came off the bench for one of my mids and then Brandon Vincent came on for the other and my formation switched to a 4-3-3. Note that Vincent provided me 2 points, whereas my other defender Sjoberg would have given me 3.