Well the drop is supposed to represent the drop you would have made had those been just regular ranked games.
E.g. you went 1-2 in the first promos at 20 LP per win/loss, so you dropped 20 LP. You went 0-2 in the second promos at 15LP per loss (since you'd been winning recently and were expected to be higher than your current place) and thus ended at 70LP. You went 1-2 in the third promos at 24LP per win and 17LP per loss (again, the game thinks you deserve to be in a higher tier) and so you get +24 - 34 = -10LP to leave you at 90.
Not sure why the games are lopsided. Maybe you just play a lot better personally because you're excited to win a promo after a good start, and play a lot worse because you're in a bad mood and think you're going to lose a promo after a poor start? Otherwise it could just be luck, not sure.
Disclaimer: I have no credibility on the last point here, just opinions.