LP To-Be-Gained was different than amount given

https://i.imgur.com/MyDxpxm.png Post-game lobby showed that I would gain 21 LP but only gave me 18. This is obvious because I was at 0 LP previously. Last game when I lost I was at 8 and went to 0 (is negative LP a thing?) Wasn't sure if it was correct, but sure enough, I check my profile and I only have 18 LP: https://i.imgur.com/0i9PKgi.png This has only happened once, thought I'd report it though.