If someone was silver and got "carried to plat" they are fine in plat, not everyone is a game changing carrier. My guess is the people are good at a specific role that fits well into premades. ie.
- very good initiating support, at lower elos people like to watch the 5 man galio, malphite, braum, morg, and nunu ults then berate the support for doing something that should be an honest to goodness fight winner.
- good vision control jgers like nunu often stay at lower elos when they get placed there due to their vision not making an impact when nobody watches the mini.
- Tank players suffer the same fate as initiating supports unfortunately (rusty after a year off before coming back, but this was my problem a few seasons ago.)
Overall, there are a lot of people especially in real role-playing positions that are sooooo much better when playing with other good people. The best honest to goodness support players have a hard time crawling from the dredges of bronze and silver. Its been a common theme on the forums for years now. "support player looking for good adc". Having a real "team style" of playing kills peoples rankings in yoloqueue and always will, because that good support/low damage jg/etc will never be able to overpower the wave of bad leagueing on their own.
note: what slayer said is also true and does happen, that's different, but if the person holds their own once theyre up there, they deserve it.