The UK DIY FAQ gives the following advice for plastering a ceiling I quote

"Ordinary plasterboard should be mounted
with the "bad" side (where the joins between front and back paper are)
downwards if it is intended to skim the whole surface, because plaster
sticks to this side better."

However on the plasteboards I have just purchased (Knauf) it is written on
the bad side I quote:

"Plaster/Decorate other side only"


What gives ?

Many thanks

Mike Saunders