Why am I getting so few LP for wins with a high win rate?

Domovoi66 - Summoner Stats - League of Legends
Domovoi66 / Silver 2 40LP / 185W 193L Win Ratio 49% / Alistar - 125W 120L Win Ratio 51%, Leona - 25W 36L Win Ratio 41%, Zyra - 36W 24L Win Ratio 60%, Blitzcrank - 1W 3L Win Ratio 25%, Cho'Gath - 0W 2L Win Ratio 0%
Not to sound like I'm whining (which I am :D ), but since I restarted League a few weeks ago I've consistently had a minimum 67% win rate, double the number of wins than losses, so why do I still get 19-21 LP per win, and why does it sometimes go down even after a run of wins? Most of the people I play with are around the same rank as me, so not sure why my LP isn't increasing with such a high win rate? Maybe I've misunderstood the system, but I thought high win rate = high MMR = more LP for wins? Or is it a lot more complicated than that!
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