There isn’t any requirement that the Hostname or computer name match, though it is customary for them to be similar. They are separate and independent names for the machine. Whatever is providing the local DNS is the most likely source for an oddball hostname. A name change made with scutil or similar tools may or may not be honored by the DNS server.

Thanks for the information. In this case the application I'm using relies on having the computer pre-registered using the computername. I was setting up an account using the hostname. However I was unable to connect upon further investigation the application is passing the computername across. I've not seen the suffix of ".modem" being applied before and didn't know why. I didn't think it could be the router adding the additional. Thanks