Daily Use

This number is a midnight-to-midnight calculation. At any given point, for a past date, the number you see is final. For a running date (i.e. today), this number will have delays based on cache setup. Cache delays can be up to 30 minutes at an individual connection level, and can go up to several hours for community level aggregate numbers.

Current Month Use and Unbilled Amount

Billing is calculated on current month usage. Totalling of monthly usage happens well past midnight and hence this will be slightly more than the daily totals of past days. At the time of writing this, the job runs at 7am (and hence carries the usage from midnight to 7am as well). This can vary in future based on load balancing choices made.

Today is part of this month too, not just days in the month till yesterday. And hence usage for today at the time of job is part of monthly total.

Monthly Bill

This job runs based on the billing cycle setup for the community. By default, this runs on first day of every month for the previous month. At the time of this job the billing month is already over and there is no partial day being added, or any ambiguity in calculating usage. At this point you will note that sum of daily totals and monthly total will exactly tally. In fact, the PDF bill you can download details this.

Hope that clears up the confusion around minor difference in running month total and sum of daily numbers till previous day.