> PYTHON_VERSION_DEFAULT in mk.conf is no longer always obeyed.
> This behavior has changed since Apr 21st.

The default version has changed. The algorithm is still the
same (see "pyversion.mk", just after "try to be intelligent":-):
1. if the default is already installed, and OK for the pkg, use it
2. if one of the other versions accepted by the pkg is installed, use it
3. if the default is OK for the pkg, install and use it
4. install the first version accepted by the pkg, and use it

> if I have python21-pth-2.1.3nb4 installed and
> PYTHON_VERSION_DEFAULT = 23pth, the pkg system attempts to build
> py21pth versions

That's case 2 above. If it worked to your expectations before,
it was by accident.

best regards
Matthias