AWStats only concentrates on the statistic logs generated by the IIS, which will only refer to domain website transaction. For HELM Control Panel statistics, it includes all transactions that is Email, FTP, and Website. Hence, the values are different from each other
For your information, the bandwidth usage shown by HELM is for the total usage of over the last 30 days. However the bandwidth usage shown by AWStats is the total usage for current month.
That's the reason there is a big gap on the bandwidth usage. You may just ignore the bandwidth usage in HELM and just refer to Awstats.