How/when is domestic usage billed?

I ask because I've noticed the following behavior...

When testing international calling, I saw that the calls were immediately billed to the account upon call completion (makes sense, since these calls can be costly, and a potential area for fraud).

When testing domestic calling, I see the fees "calculated" in the call history tab, and I've seen a few 'Usage Charge" line items appear in my billing tab - one for each day there has been calling activity on the account. However, the "Usage Charge" bills at different times of the day, and the charge in the billing tab does not always match the total of the fees in the history tab.

HTML Code:

For Brandon & Tim's reference... Date History Total Usage Charge Billed 01/20 $0.00 $0.62 (NO domestic calling this day?!) 01/21 $0.01 $0.01 01/25 $0.04 $0.04 01/27 $0.11 $0.10