Help me understand how the points system works

I have just been promoted to brz 4 and had a good run of victories but then came the obligatory feeders and trolls and i was back down to 0 points again. I expected to go str8 back to brz 5 again but i had not taken much notice of the points to be honest. when i reached 0 points, i wanted to check up on them, so if you check my match history from the last game of the 5th july TRYNDAMERE to the first 3 on 6th july ending with SONA, you will see i won three games and got a total of 21 points. Then i lose **_ONE_** game and lose 21 points and i am back to 0 points again. How can this be fair? how can this be considered even the slightest bit encouraging to continue playing the game? Three hours to climb to 21 points and 29 minutes to be back where i started. help me understand PLEASE!!
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