That's pretty weird, I would try entering a new Computer name inside Sharing Preferences, and then entering the name you want after. I've found that sometimes this updates the hostname if the previous one didn't for some reason.
This sounds like a bug though, I would report it to apple.
The computer name and the username are not the same.
Typing 'hostname' in a terminal window shows my computer name as seen in sharing.
The command prompts in my Terminal window show my username (short name) at each and every new prompt line.