Pretty unsure myself, I would say it is a total of recent days , or weeks? .... the offset log seems to have a grand total of previous bytes_logs, just a single number that Cpanel's accounting shite interprets as... something.

Hahah, in other words, it's probably nothing earth shattering and it looks like a &running total& for the domain in question- though I am unsure as to how those numbers are interpreted by the backend software.