Can someone explain to me whats happening?

Ok so there is a lot that i don't understand and make no sense... First of all i am silver 1 and i am trying to get to gold... Can someone plz explain me why when i won this game it says +18 but i get only 8? out of 18 as you can see here in the picture http://oi66.tinypic.com/dyvvnt.jpg ... I had 0 yesterday so there is NO WAY i was in a negative score or anything like that because i ranked up and didn't play another game after that.

This is not the first time something is off with my lp... Sometimes when i lose a game i am for example 79 lp and i go down -18 so my new lp is supposed to be 61 right? Wrong.. It was 45 for some reason.. Its like i lost 2 games or something..

Not only that but whenever i win i game i get 18 lp and when i lose -20.. This happens all the time and i checked my mmr on op.gg and its 1,426 which is normal for silver 1 players.. So why do i only get +18 every time i win?.. and we are not finished yet..before some days i had 1500+ mmr but i was getting the same lp again so there is no explanation.. I don't understand why riot is trying to derank me so hard because i am trying my best and i always get an s.. s+ and at least an A when i lose with not a negative score but i get shiit like that all the time.. Just take a look at my history most games i carry the whole team. This is so frustrating i rank up so slowly because i only get +18 and sometimes when i lose i lose double lp.. like wtf?.. Silver 1 is not my rank. I am expecting an explanation with the lp problem at least.