Its because of how the system is set up and how elo is gained and loss vs who you play against. A 45% winrate diamond player can in fact still be better than a 60% winrate silver player.
First off Winrate doesn't = skill entirely
Second when you hit different tiers you start facing opponents of different skill levels, so you could have been winning games hard and fast to climb quick giving you a HIGH % winrate at lower elos as you zip past them, but then once you hit where you and other players you face have similar skill your winrate starts to even out.
Now because of how the system works your elo / lp gains and drops get skewed at different thresholds, A silver player facing other silver players will have lower lp losses and gains than say if a silver player faced gold players, this holds true for all the elos, so a diamond player is facing (hopefully) other diamond players so if they are losing enough to gain 45% winrates it doesnt mean they are the same skill of a silver 45% winrate player, they are still probably better its just they are facing a different class of people.
And in EVERYGAME someone has to lose, which means there has to be kind of a leeway of lp / elo both in negative and positive %winrates 45-55% isnt really that far fetched to STAY in your tier / elo thats why 55% winrate diamond players are still diamond AS WELL because there hitting those lp and elo gains just enough to stay but lose enough to not climb, and same for a 45% winrate, they could be gaining enough lp to stay but losing enough to also not climb.
have a slightly negative or slightly positive winrate doesnt mean you "Deserve" to be a different tier, it just means you are facing players of similar skill and losing or winning a bit more than them. this is why you still have 45% winrates in ANY TIER without demotion and also 55% winrates without promotions