You can see Current Hostname is (server.domain.com) while in cPanel you can see Server Name is (server)  and not the actual server name.
If you want to change the hostname you can do,it from the below  :-

Ans :-
This is the intended behavior for the side bar in cPanel. The “ServerName” statistic will display the name before the domain name of the hostname. For example, if the hostname of the server is “server.domain.com”, then the “Server Name” will output “server”.
You can use the Branding Editor to edit the “extended_statsbar.html” page. Simply change “shorthostname” to “hostname” and it will output the full hostname in cPanel. This option is found at:
“WHM >> Branding Editor >> Edit Branding Style >> Edit HTML Pages”