[gentoo-user] PYTHON_SINGLE_TARGET inconsistent?

[Andreas Fink]

Hello,
I have a question considering PYTHON_SINGLE_TARGET because it seems to behave
inconsistently on my system and I cannot find any documentation about it, that would
guide me in the right direction how to fix it.
Running emerge --info I see the following (I'm on ~amd64):
... PYTHON_SINGLE_TARGET="python3_5" PYTHON_TARGETS="python2_7 python3_5" ...

This output is the default for ~amd64, neither PYTHON_SINGLE_TARGET nor PYTHON_TARGETS
has been set explicitly.
If I do an emerge -av asciidoc, I see the following output:
ebuild   R    ] app-text/asciidoc-8.6.10::gentoo  USE="-examples -graphviz -highlight {-test}" PYTHON_SINGLE_TARGET="python2_7 -pypy" PYTHON_TARGETS="python2_7 -pypy"

Setting PYTHON_SINGLE_TARGET and PYTHON_TARGETS in /etc/portage/make.conf explicitly to:
PYTHON_SINGLE_TARGET="python3_5"
PYTHON_TARGETS="python2_7 python3_5"

Now emerge --info doesn't show any difference. It's still the same output.
Doing an emerge -av asciidoc I get now this output:
!!! The ebuild selected to satisfy "asciidoc" has unmet requirements.
- app-text/asciidoc-8.6.10::gentoo USE="-examples -graphviz -highlight -test" ABI_X86="(64)" PYTHON_SINGLE_TARGET="-pypy -python2_7" PYTHON_TARGETS="python2_7 -pypy"

  The following REQUIRED_USE flag constraints are unsatisfied:
    exactly-one-of ( python_single_target_pypy python_single_target_python2_7 )


This seems for me to be really inconsistent, considering the fact that we usually request users to provide an emerge --info,
which wouldn't show any difference.
Why am I able to merge in the first case the package, but in the second case I get an
error? I really cannot understand where the python_single_target_python2_7 is set in the
first case.
Anyone who has more insight into that case?

Cheers
Andreas