Mastery score the same for all teammates?

I just played my first game since mastery was introduced, I also played with 2 friends who was in the same situation. Anyways, we played our game and we won it, after the game it showed that I got a S, while my friends got a A and a B. Here's the thing though, when we later went to our summoner profiles, the amount of mastery points we've gotten were exactly the same, 1264/1800 - Level 1 for all three of us. Wasn't it supposed to be so that a higher score gives more points? Or is it so that everyone in the premade group gets an average amount, in our case an A for everyone? Or have I just completely missed something?