20.393 style:writing-mode-automatic - misplaced text? see description

Reads in part: "The actual value for the writing-mode should be contained in a style:writing-mode attribute, so that consumers that do not support an automatic writing mode calculation or use a different algorithm always know the actual value."

Shouldn't this (with some re-writing) appear under 20.392.1, the general section for style:writing-mode?

Or to put it differently, why does the attribute value become a "should" under another attribute?