I'm so confused. How does the ranking system actually work? :s

So I just recently made gold for like the third time. This season I had my first ever bronze experience, it was actually fun, I carried the shit out bronze winning more than 70% of my games there, started mid bronze, barely took a week to climb out of it. I almost miss it, roflstomping people was quite fun for a change.

Either way moving, on, got into silver, spent quite alot of games there, my winrate plummetted from the whopping 70%+ to a more reasonable 55%. I've now increased it to 57% (part of a very long win streak and the reason I'm now gold). So here I am sitting in gold at 57% winrate. Nothing impressive admitable, and I'm happy to be here once again. I have my specific champs that I do well with and I avarage above 60% winrate with them and hovering around 3.0 kda on avarage, yes I know this also isn't anything impressive but it's not utter shit considering I play solo.

So as to the confusion part. I just partook in a discussion, and as I've been tought through boards, when someone doesn't agree with me, look that sucker up on op.gg! GG. :D

So I did just that and noticed a player, who is platinum 3, on a 40% winrate, with abysmall stats on every played champ and has just about the worst kda I've ever seen, nearing sub 2.0 and I ask myself, how is this even remotely possible? Shouldn't this player have at the very least plummeted to the bottom of platinum 5? Or did this player perhaps start in diamond and worked his/her/it's way down from ther?

This isn't a rant or anything, I just genuinley want to know how the system works.