GCPM "float-offset" and centering

Although I was resisting "float-offset" definition initially, I have to admit it is brilliant. It handles all cases of facing pages and writing directions in the most minimal way.

One statement in the definition of float-offset I disagree with so far (or perhaps I don't quite understand its meaning):

"If the 'gr' unit or percentage unit is used, it means that the middle of the float should be aligned with the specified grid line (or portion thereof)."

I think this mixes two different concepts (positioning and centering), and even in the use case table (http://www.w3.org/Style/Group/css3-src/css3-gcpm/uc.html) "gr" usually just means position, only in case #30 it means centering.

Centering a positioned element is a great feature but maybe it should be a separate property? What do you think?