The behaviour in this area is different between 3.7.0, 3.7.2, and 3.7.3 (at least). I have reproduced the issue with 3.7.3. Steve Dower made changes to the way the python executable works in venvs in the point releases of 3.7 - see https://github.com/pypa/virtualenv/issues/1380 and https://github.com/pypa/virtualenv/issues/1339 for some discussion of how this affected virtualenv.

I suspect this issue is related - from 3.7.2 onwards, the python.exe in a venv is a redirector which runs the "base" python.exe, but with sys.executable showing the redirector rather than the actual running exe.