I don't think that "overhead" is counted either.
The "usage" shown in the Hourly HISTORY REPORT is "Gross Usage".
The report is useful for tracking when activity is occurring but in many cases the entire amount is not necessarily deducted from your allowance.
For that, you need to look at the "usage meters" of which there are three:
The one in the modems SCC main page
The Download Status Meter desktop utility
And the most accurate and detailed ..this one: