ISPConfig and your provider are measuring something different.
ISPConfig shows you the traffic that has been logged by apache in the logfiles (web) and proftpd (FTP) and postfix (Mail).
Your provider measures the IP traffic that goes trough your network card.
If you dont have other services running on your server that are not measured by ISPConfig and the 76 gig are protocol overhead, i recommend to scan your system with a rootkit checker like rkhunter (
www.rootkit.nl)