Here's a thing.
The system keeps track of more things than just KDA, gold and cs. There's fighting, vision, income, aggression, consistency and a bunch of others (I suggest checking tracking sites such as Mobalytics or Blitz as they use them in much more detail than what OP.GG does).
Since so many things are taken in consideration, they are compared to average players in your current rank, sure, if your KDA and CS are up to what they get, but then your consistency, vision, aggression, fighting, objectives, etc are all much lower, the system will still see you as being above the rank you should be, and when that happens, you get lower LP ganks and higher losses, it's not to force a 50% wr, since you can still climb, it's just that your needed stats are lower, and if what you say is correct, that you get around 14, they may be much lower than needed.
I consider my performance average if not lower than average and if you consider only cs and kda, I would most definitely be considered boosted by some players, and was even called that, I'm in Gold, yet I gain 21-22 per win with 17-18 per loss, so despite me seeing myself as doing poorly, the system has a different opinion.
I don't focus on getting the 200-0-12 KDA and be the "one man carry", sure, sometimes I get free kills and use them, other times I don't, my focus is on winning so I'll always take actions that benefit that, and a good bunch of the stats tracked all can combine into that to show it: map presence, neutral objective, building damage, kill participation, vision, durability, teamfight positioning.
Point being, there are so many more stats tracked than just KDA, CS, Gold and such, for gold alone it's being tracked in 6 different points: early compared to rank, early compared to corresponding enemy, mid game compared to rank, mid game compared to corresponding enemy, late game compared to rank, late game compared to corresponding enemy.
Another thing to keep in mind is consistency, it doesn't mean going just 20-0 every game, it's an average for a period, so if you go 0-20 every game for a period, the system will still give you a high consistency score because although you're feeding, you're consistently feeding every game, and that is used to determine if a player is too low or too high, if he's consistently stomping (again, even in consistency there's more stats tracked, not just kda, but makes it easier to explain) then the system will give more LP to decrease the climb speed, if he's consistently feeding, the opposite happens.
Same may be for you, although your KDA and the obvious stats may be "good" or "ok", if everything that's not easy to see (especially because very few trackers use them) are low, and low consistently, then the system will give you less per win and more per loss because you are consistently underperforming.
Hope this helps.
