Guild Wars 2

After I winning a match, I ended up gaining 1383 or so rank points instead of the usual ~1500. Does anyone know what could be causing this?

Yes, their team was down a player for a limited time is one possible cause

Was it a blowout match, where you won very fast?

You have to be in the match for at least 7 minutes in order to get full rank points. Before that is diminishing returns. If you win in 6 minutes…you get about that many..5 minutes you will only get about 1k. So on and so on.