Why USE_PYTHON=yes doesn't rebuild default python meta-port?

[Yuri]

I am rebuilding the port hierarchy with the top port having 
USE_PYTHON=yes. This package runs python-config.
But only python2 and python27 were actually built, and therefore only 
python2-config is available and my port breaks.
Default meta-port python-2.7, that would have contained python-config, 
wasn't built.

But USE_PYTHON=yes asks for the default python, doesn't it? So is this a 
bug in the make scripts, or else why default python version wasn't built?

FreeBSD-10 with current ports tree.

Yuri