Gentlemen, and I mean that is the most friendly way. Honestly I do.

I
DO
NOT
CARE !

The end result in Poser SUCKS. :-(

All this is mathematically completely correct.
And you are all 100% correct.
But the end result for Poser users SUCKS!

If this is the future?
If this is what SubD brings???
I'd rather not see it in Poser.
Thank you.

@ Paloth
Again, mathematically 100% correct.
That is why I set Posers Smoothing to 180° crease angle and render with Smoothing ON.

If that does not solve the problem? 
The problem lays deeper. As is in this case.

Poser 1, 2, 3, 4, 5, 7, P8 and PPro2010, P9 and PP2012, P10 and PP2014 Game Dev
"Do not drive faster then your angel can fly"!