Specifying the vertical positioning of the kihon-hanmen

Before I ask my question, I just wanted to say thank you to everybody
involved in producing the "Requirements for Japanese Text Layout" note.
It is an extraordinarily interesting and helpful document.

The note says [1] that the vertical position of the kihon-hanmen relative
to the trim size is either centered or specified using the head (ie
top-margin), for horizontal mode, or foot (ie bottom margin) for vertical
writing mode.

I am puzzled by the vertical writing mode case.  Given that the inline
progression direction is top-to-bottom, what is the logic for specifying
the bottom rather than the top margin in this case?  Do you specify the
bottom margin even if you have horizontal rather than vertical running
heads/foots?

James

[1] http://www.w3.org/TR/jlreq/#procedure_for_defining_the_kihonhanmen at