Are Global Variables and Custom Properties the same thing to SW?

I received this error message when trying to create a Custom Property "LENGTH":

"A Global Variable with this name already exists. Adding a Global Variable with this name would cause a name conflict that may cause invalid equation results."

I had just created a GV "LENGTH" and wanted to link it to a CP named "LENGTH", but it wouldn't let me.

Which is weird because I had just done this in another model.

So I checked my workflow and sure enough if you add them in the wrong order SW freaks out.

I have successfully added both by adding the CP first, then the GV, and then linking the CP to the GV.

I thought I could save a step by adding the GV, then adding the CP and linking it to the GV right away, but NOPE! SW hates that idea.

Bug?