i think the case worker might have meant it was not on proper headed paper
as i've worked for printing companies, i know, esp with the colour lasers now and editing s/w, its so easy to produce a document or certificate that looks real .
and your right, i've scanned every doc and cert i've sent to the embassy, and its really difficult to tell which is the original
, i've even managed to copy my misses IELTS cert , even thou they've used anti photocopying/scanning techniques on it, (greenish colour, lots of gray scales) and if i had decent paper you wouldn't know which was the copy or original.
to others, always have a copy of anything you send, especially originals, you might never see them again