Calibre CALIBRE_DEVELOP_FROM oddity

I run calibre from source most of the time however I wanted to test an issue and so changed the variable name to not run from source. This worked until I closed calibre. The next time I opened calibre, it ran from source. I check the environmental variables and the variable name was unchanged however when I ran set from the command line, it showed both the original and modified variables.

Is this expected behaviour?

CALIBRE_DEVELOP_FROM=D:\calibre\calibre-master\src

DNU_CALIBRE_DEVELOP_FROM_DNU=D:\calibre\calibre-master\src