Whoa, now, boys; how 'bout a deep breath or two? I am
sticking my nose in without a paddle (gee I hope that made you smile ), because I've not come across this word before,
but going by the quotations, it looks to me like 'ullage'
can be used to mean both: the amount that is there,
AND the amount that isn't. Witness:
The quantity of liquor contained in a cask partially filled and the capacity of the portion which is empty are termed respectively the wet and dry ullage.
So, perhaps users of this word need to be specific.