Make `hostname -f` return fully qualified hostname
OS X is different in this case in that it might change your settings when it gets a response from your router / DNS+DHCP. 

If you aren't getting a FQDN out of hostname, you couod set a FQDN first.

The Mac tool is scutil

scutil --get HostName            # same as hostname
scutil --get LocalHostName       # same as hostname -s
scutil --get ComputerName


If you want jvf-imac.fritz.box then simply:

sudo scutil --set HostName jvf-imac.fritz.box


You could set the host name with sudo hostname ... as well and it wouldn't be wrong. If I were to guess, you probably have fritz.box in /etc/resolv.conf which is why the DNS lookup appends the domain name since you didn't specify one more fully.