what those people said is wrong.
The ranking system is based on the chess ELO rating system. that basically means, everyone started at 1600 and the better player you beat the more your ELO goes up (rank)
so say you 1601.53 and you beat someone with a 1645.12 your ELO will go up a lot more than beating a 1602.01
Your rank changed so much over night because it was updating all your wins, and everyone else's
rank is based on your ELO 1708.99 ELO will probably make you top 10 players. Don't rely on rank so much
Conversely, if you lose to someone with a low elo (1601) then you will lose more ELO points than if you lost to someone with a similar ELO. The same applies to your rank. Also, these changes aren't updated in exact real time, so most likely what you are complaining about is other people winning more and against better players, so they rise in rank faster, thus pushing you down.
tl;dr Nothing is wrong, just realtime update lag.