As most everyone else said, a lot of it is people avoiding the sense of pressure of Advanced, while going toward Advanced standards more (or completely). Technically, in Casual, you can still post by the actual standards there, it doesn't have to be Advanced - where in Advanced, you have to meet its standards always.

It's also "cultural." Advanced has the expectation of veterans that have RP'ed often for years, and well. Casual's got a broader range of types of players, or there's less of an expectation. Even though Advanced's standards are surprisingly low, the infamous length requirement being some two paragraphs, the standards look higher because players feel the pressure to meet the same level of work as other players.

High Casual implies stricter policies and more effort than "anyone who can put together a few sentences," a small standard, but takes off the expected pressure of Advanced. Or, that's the idea. It generally ends up having the same expectations because of the same vicious cycle that Advanced gets, where some players do long posts, and other players want to meet that same amount of work.