DOCBOOK: curious about attribute default on <itemizedlist>

from the published version of docbook: TDG, just as one
example, an <itemizedlist> has an attribute of "spacing", 
with possible values of compact or normal, but a listed
default of none.  what does this mean?  how can it not have
a listed default value?  just curious.  i've noticed other
synopses that are similarly confusing or contradictory.

rday