It does work differently in the school of music compared to other schools and departments. This is a degree denoted as a DMA rather than a PhD and focuses on creative activity instead of the standard research that would be part of PhD programs. In most (if not all) programs in the US, the DMA is the equivalent to the PhD for the music composition degree (and most programs offer the DMA instead of the PhD). In other degree "subfields" in music, the degrees are either a DMA/DM (performance and conducting for example) or a PhD (theory, education, and history for example). The DMA is a relatively new thing, making it possible for professors in the "creative/performance" side to get an equivalent degree to those in the "standard research" side.

The requirement of having all the coursework done before achieving candidacy only pertains to the required coursework and not to the full 90 hours. They push us to complete the entire degree in 3 years and to have all the 90 hours done before candidacy, I'd have to have gotten it in 2 years. Also, studio lessons are required every semester (including studio class) and possibly recital credit so it's possible to have 9 hours even without and actual "class". I think most people do that as their degree allow for them to take more of their lesson and recital hours for degree credit. I have to take more electives for whatever reason. I may be able to finish my 90 hours without a typical 3-hour class this semester, but it's possible and common to be taking classes your final semester (in my department anyway).

My dissertation is a composition and analysis, so it isn't the typical dissertation you're going to find in a research PhD.

I don't know if that makes it any clearer? I realize that music departments do things vastly different than other departments, but some of that is likely the nature of the subject.