The 8.5kg fill is because they fill the cyl by weight. They do not weigh the cyl first, so by allowing a 500g variation between lightest and heaviest cyl, 8.5kg fill will ensure no bottle is filled past the 80% level.
Now for the tricky part with decanter type filling from the bulk cyl. as the temp rises, so does the liquid volume, but not weight, the big decanter cyl sitting in the sun is growing $$ worth of gas every degC hotter it gets, so a fill to the liquid reaching the upper level where the vent extension reaches may be 18ltr, but not necessarily 9kg. The person who sells decanted gas by weight, weighing the bottle before and after is the very rare honest seller.
Don't get me started on the verification of the Aust standards of 9kg and 4.5kg capacity gas bottles, let's just say, the collar on the bottle is welded on where it is produced, already stamped, complete with inspection number, yet the inspector never leaves Aust.....
as an interesting exercise, take your portable scales to a shop selling new cyl, read the weight marked on the collar, then weigh the cyl, try a number of them, see if there is any relevance between what is stamped on the cyl and what it actually weighs
