How has the level difficulty rating system changed over time?

I haven't played much Geometry Dash since 2016, and a lot of my knowledge of what the game has been like since then comes from GD Colon videos. And in these videos and various comments, I've heard that the way levels are rated for difficulty has changed over time; for example, for a level to be rated Easy it has to be easier than in the past, and levels have to be harder than in the past to be rated Demon. My question is, how did this happen? How was it possible for a large community to collectively decide to rate levels differently? Was it deliberate or accidental? (I don't really understand how the rate 1-10 system relates to difficulty ratings, or what it means for a level to be "rated", so it would be helpful if someone could explain that too.)