I think the whole thing is just perception. People consider themselves above the title "Free" because it sounds lazy, while "Advanced" sounds intimidating. People RP as a hobby, not a career, so "Casual" sounds more accommodating. But the description of Casual is "at least a paragraph or two, character development, and some depth. Grammar and spelling are encouraged. Generally one paragraph (a few sentences) per post." That sounds low-effort, which is discouraging for people who put a relative amount of work into their RP (enough depth and development for a stable role-play with a preference for two or three paragraphs per post). Hence the "high-casual" tag; people who want to write at a high standard, but are worried that they just don't measure up to the title "Advanced." 

I've seen RPs in Advanced that have 10-15 paragraphs per post. I can't imagine writing that much over and over when I also have a job and obligations in real life, so I generally avoid it. But the Casual description says "generally one paragraph per post." So where do I go with my general two to five paragraphs?

I think a large problem, therefore, is the description of the sections. Instead of Free, Casual, and Advanced, they sound like Free, A Little Better than Free, and Advanced. There are many members here who write at an Advanced level and don't even realize it because the imaginary Advanced bar is so high. 

I'm in a Casual RP and an Advanced RP, and I write at the same standard for about the same number of paragraphs (two to five) in each. And it fits just fine in both of them. But you wouldn't think that from the descriptions of the sections.