Generally they don't want to write it down & won't give a 100% answer because they could be wrong....With written proof of them saying "it's a girl (or boy)" they can be sued if it turns out to be wrong.
Yeah, at our (private) hospital they won't even tell you what they think the sex is as a technician got it wrong once and the hospital was sued! Kind of spoils it for everyone else who wants to find out the sex.
It doesn't really worry us though as we are happy to wait until the birth the find out the sex of Beany.