It is NOT the number of games you win, it is the quality of the PLAYERS you beat. If you win or lose to a team of 4 Bronze and 1 Plat, those 4 Bronze are going to affect your MMR more than that 1 Plat did.
Also, losses are going to hurt more than the amount that winning gains you. Why? Because of the belief that it would be better to be put in a slightly worse rank that you could easily climb out of due to you being better than to accidentally put you in a rank actually way out of your experience and you just drop anyways.
Lastly, the guy who said it a while ago was right, your first couple games matter more than your last ones. Your early ones pinpoint your MMR to where you should be, your later ones serve to fine-tune it. Basically put, if your 3 losses were against Bronze players then the game dropped your MMR hard, especially since it was in your first 5.
Riot has said this numerous times lol Honestly, it's smart, I myself would much rather quickly climb out of Bronze than get my ass handed to me in Plat until I'm knocked out or whatever.
Personally I'm waiting a week or two till the diversity of everyone playing has been spread a bit better, after all the people in Plat and above have likely already finished their placements.