Why when I win a game I get between 15 & 19 lp, but when I lose +20 lp is taken away..

I just don't understand why this happens to me I even checked my account MMR in websites (like lol-mmr) they say that my skills mmr are silver and my elo is silver so that means that my skills = my elo... So why when I win the system give me just 15 or 19 lp for maximum and when I lose sometimes 22 lp is taken away. (sorry for my English) ps: Sometimes when I win I gain 22 lp and when I lose, I lost just 17 lp ( but that happens just sometimes) I don't know why but the system is just random.
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