Exponential distribution of skill

The closer you get to the top, the bigger the difference in skill. The difference between a pro and a random masters player is likely to be the same as the difference in skill (in terms of relative win rates against each other) as the difference between bronzes and plats. A diamond 3 has a 90% win rate against a diamond 5, which is about the same as the difference between a diamond 5 and a gold 5.

This doesn't mean, of course, that plats = silvers. The difference between a plat and a silver is unimaginably vast. The silver has almost zero chance of beating the plat. However, the plat likewise has almost zero chance of beating high diamonds even though in rank they are only separated by 5 divisions or so.