python sys.executable inconsistency if called from cmd

[Lemke, Michael SF/HZA-ZIC2]

I don't know anything about python but want to use cygwin's
python from nodejs with npm-gyp. The npm-gyp module is trying to
find the python executable path from sys.executable and fails.
While investigating this I found this to me quite inconsistent
behavior:

cygwin> /bin/python3.6m.exe -c 'import sys; print(sys.executable);'
/bin/python3.6m

cygwin> python3.6m -c 'import sys; print(sys.executable);'
/usr/bin/python3.6m

cygwin> python3.6 -c 'import sys; print(sys.executable);'
/usr/bin/python3.6

cygwin> python3.6m.exe -c 'import sys; print(sys.executable);'
/usr/bin/python3.6m


cmd> c:/MyStuff/NCygwin64/bin/python3.6m.exe -c 'import sys; print(sys.executable);'
/usr/bin/python3.6m

cmd> rem add cygwin path to Windows:
cmd> path %PATH%;c:\MYSTUFF\ncygwin64\bin

cmd> python3.6m.exe -c 'import sys; print(sys.executable);'
/c/MYSTUFF/ncygwin64/bin/python3.6m

With CMD I'll never get an executable that actually works from cmd.
Is this expected? Is there a way around to make this work?