Title says it all... have 98 points, win another and go into series. I lose the series (go figure, teams feed lanes, dive 1v3 thresh into our 100 hp tower... stupid stuff like that)
So, I go to check my points? 58 points. Why?
Because each loss in a series deducts points as normal.
So for a normal series it would be 21 points per loss, and for a Bo5 it would be a 14 points per loss.
Riot doesn't want you to go up in rank. Plain and simple. That extra 15-20 LP you got for that win went up in smoke, but you better believe LP lost at 0 LP to a drop isn't gonna disappear
There is a thing called HIDDEN MMR
Your LP losses and gains are trying to pull you towards that MMR
