The ranking system is based on elo. The higher your elo, the higher the rank you are likely to have. Per example, if you have an elo of a little over 1660, then your rank is likely to be somewhere in the lower thousands. If your rank was 1,500 something, and your elo was 1661, then you beat a player with a rank of say 1000, and their elo was 1662, then you would automatically steal their rank if you beat them because your elo was extremely close to theirs. Now if you have a high rank and you are constantly playing against people that have an elo that is extremely lower than yours, then your elo will drop even if you win because of the difference in elo.

At-least i'm pretty sure that's how it works, they still refuse to change the ranking system either way.