I can't say as this is something I've ever noticed. I'd imagine it entirely comes down to that person's own definition of what makes a cred.
I don't think I'd ever go so far as to put 1/2 on my actual count. However, I do class each side of a Möbius coaster as two halves of the same cred. Therefore, if for some reason, I only managed to ride one side, logically I should only count that as half a cred.
I imagine there are a few people that maybe count the two sides of a racing coaster as two halves of the same thing, potentially leading to the same issue.
I don't know, someone will probably now come along with some elaborate reason for their 1/2... but I'd guess that the vast majority are variations on the above.