The system is based on two ratings: An invisible matchmaking rating (mmr) and a visible league/division/lp rating (lp). Your mmr will change significantly each game with the ONLY statistic that is important: did you win?
You cannot take k/d/a or buildings destroyed into consideration, you could have been carrying by distracting the entire enemy team and dying for it. You cannot take leavers into consideration, you'd tell people to go and make others leave to lose less mmr/lp (even if they got reported for it, there'd be this perfect balance of flaming/trolling which doesn't get you punished yet does get the most players to leave to optimize lp/mmr gains).
Your mmr will change significantly each game, even if you haven't improved significantly. If you get an unlucky 5-loss streak, your mmr will drop like a brick. This is where lp comes in. Your lp will try and follow your mmr, but it will lag behind. So when your mmr drops due the 5 games, your lp will follow, but slower. Then when you get the 5-win streak because you were better than those with whom you played due to your loss streak (your skill never changed), your mmr will rise back up to the same value. Your lp will follow your mmr once more, slowly. Your mmr will now have gone through a significant drop and rise, but your lp not so much, nor did your actual skill level. If you then go on with 100 games where win and loss streaks happen all the time due to the rng, your lp will stay relatively stable even though your mmr fluctuates significantly.
Now, if you have improved, you will suddenly win 6 out of 10 games. Your mmr will slowly rise and keep rising. Your lp will lag behind, but when it's too far behind, the lp gains will improve more and more. This means your lp will be following your mmr not too far behind at any given point. Your mmr will resemble your actual skill level before your lp does, but not by too many games.
This means that your lp is more accurate in total. It will not fluctuate too much during random streaks, but it will resemble your skill level if it improves.
The only thing stopping it from doing so are promotion series. Promotion series are horrible for lp accuracy because they just hold you up until you get a random win streak at which point they let finally let your lp through while your mmr was just steadily progressing all this time getting quite a lead on your lp (which is then inaccurate).
So if your lp gains are low, it probably just means your mmr is lower than your lp. If it doesn't accomodate for leavers, it is to not promote bad behavior. Just stop caring about any single game, it's about the bigger picture, improve your skill and your winrate will be over 50% over 100 games, this is the most efficient way to improve your lp.