Someone explain this ranked LP math!

Ok so I lost a ranked game, no big deal I had 19 LP and it tells me I lost only 16. So I should theoretically have 3 LP left correct? That's how real world math works! But apparently that doesn't apply in League (or its just bugged) cause now I have 0 LP after going from 19 to losing only 16. Where is the missing 3? What am I missing here?