Are XP Amounts Correct?

I had a game earlier today where, after 30 minutes, my team won our game. I went 11/2/8 as Nasus with 253 CS, earning an S+. Yet, I only received 50XP according to my Match History (I don't remember what the postgame summary told me). Is this correct, or is it the bug where XP given doesn't display properly? I've been confused for awhile now regarding how XP given is calculated, as it seems to fluctuate greatly between games, and I know that many of my friends feel the same way.