To reproduce:

> py -m venv fooenv
> fooenv\Scripts\activate.bat
(fooenv) > python -c "import sys; print(sys.executable)"  % This is correct
C:\Users\uranusjr\Downloads\venvtest\Scripts\python.exe
(fooenv) > python -q
>>> import subprocess
>>> subprocess.check_output(['python', '-c', 'import sys; print(sys.executable)'])
b'C:\\Users\\uranusjr\\AppData\\Local\\Programs\\Python\\Python37\\python.exe\r\n'

The output shows the base interpreter, not the interpreter in venv. Not sure whether this is a venv or subprocess problem.