Being placed lower than you (really) actually are can sometimes be a potential blessing in disguise, since going on a 10+ game win streak makes your mmr (and in turn, LP gains) go nuts, to the point where it'll overshoot and you'll get matched with people who are (really) better than you and you're expected to lose. If by small miracle you win, then there's sweet MMR gains to be had which makes division skipping quite a real possibility, and if you do lose, then at least your mmr is closer to where it 'should' be then without the win streak.
(This is admittedly silver lining, and obviously placing higher/closer to where you 'belong' is ideal, but the ability to overshoot can be fun as you briefly get matched with people with shinier borders then you feel you deserve.)
As for the 'why' though, your placement is entirely based on your final mmr. Placement matches are particularly influential on your MMR, but your MMR going in is still a pretty big influence, and also the MMRs of those in your game matters. If by the end of preseason your MMR had tanked to low bronze 5, it's potentially possible for someone to even go 10-0 and still place bronze 5 if their MMR was just that awful going in. Similarly, if you group with a bunch of low bronze MMR people, you may easily carry the games and win them all (as you're correspondingly matched vs bronze MMR groups), but the MMR gains would be minimal since you were expected to win them. If instead you group with a bunch of high gold MMR people, even going 5-5 could result in a bigger MMR increase than the 10-0 with bronzies.
In-game performance is not a factor for placements (or mmr determination), so going 10-0 despite personally having 1 cs/min and 0/10/0 kda in every game (maybe you had a diamond smurf in your games or something) is exactly the same as going 10-0 with 10+ cs/min and going 30/0/10 hard carrying every game. The champion mastery system would give you D-'s in the former case (or whatever the lowest is) and S+'s in the latter probably, but the champion mastery system is entirely separate from the ranked ladder.