+17lp - 21lp ?

If your average MMR of the games you play is lower than the expected MMR for your current rank/LP, then the ratio will decrease the gains while increase the loss. This also works vice versa if you're average MMR is higher than the expected MMR of current LP/rank (Especially shown by smurfs and how people can skip ranked tiers entirely if it's that significant).

And easy MMR boost would be to duo with a higher MMR person than yourself to fix/bolster your gain/loss ratio (But will also be more difficult as climbing should be).

If you duo with lower mmr buddies, the games can average out to be better for them while worse for you. Edit : Forgot to explain why for this section. Example : Player Z Gold 1 with MMR of Gold 1 20+/20- Player X Silver 4 with MMR of Silver 4 20+/20- If they duoed, the MMR of of the game would average somewhere at Silver 1/Gold 4. This would be more difficult for Player X but easier for Player Z. The result should also show that Player X has high gain/ lower loss while Player Z would have lower gain/ higher loss regarding LP rewards.

Another instance is just a small explanation of two people with same rank/LP but different hidden MMR. Example : Player A : Silver 3 with MMR of a Gold 2. Ratio might look like 28+/13- Games Player A must deal with : People in Gold 2 area of difficulty. The player is versus higher people and shows that the level of play (MMR) is higher than what the Rank/LP is shown so the gain/loss is in favor of the player to get the player's rank/LP to match the MMR. Player B : Silver 3 with MMR of a Bronze 1. Ratio would look like 14+/23- Games Player B must deal with : People in Bronze 1 difficulty. If he's having a 50% wr at this level then the player clearly shouldn't be in Silver 3 which is why the gain/loss ratio looks like that.

You can find MMR estimates with sites like euw.whatismymmr.com [Unfortunately op.gg has removed theirs probably because theirs is outdated]

People can spend hours on end debating "That's not my real rank" or "My friend has a better one but we're the same" but at the end of the day, the results of the hidden MMR don't lie.