CSS variable syntax

I suspect there are many people like me that are just now learning about
the syntax for CSS variables even though it has been defined for some time
now. Many of us are going to be looking for a justification for why the
syntax for setting a variable is different from the syntax for accessing it
... why it works fine to use the same syntax in LESS and Sass, but not in
CSS. Can someone point me to a web page that describes the reasoning?

-- 
R. Mark Volkmann
Object Computing, Inc.