LP dropping on win streak???

So i understand the basic concept of MMR and LP gains i believe. However, I have only lost 1 ranked game in my last 7, two games ago i received 18 LP for a win, then the very next win I only was given 16 LP. I was under the impression that when you played poorly and "tanked" so to say, your LP reflected what division you should be in, receiving less and less LP if you were playing poorly in a higher division. In my last 15 ranked matches I have 12 wins 3 loses, not really understanding why my LP is decreasing for wins. Wonder if anyone could better explain this to me. Or is it just the wonky system in play.