var(-- sounds like the person who writes the parser was either too lazy or sucks at his job so they asked the spec writer to give them a break.

I'd be interested in a link about this (can't find one from a quick look), but guessing it's to do with avoiding clashes with names in existing tools/code and backwards compatibility in browsers. In Sass, you would just write $primary-color, but if they copied that syntax exactly, it's likely going to get confusing for Sass tools if it's a Sass variable or a CSS variable.