I know how the solo mmr system works but why am I gaining less and less lp per win when Im not losin

I just want some clarification here because I won my past 4 games and ive been gaining progressively less lp.I won one and bam 21 then next one 19 next one 19 this one its 18 and im confused because its obvious I shouldnt be bronze 1 I keep on stomping and winning...