"Sales Tax Collected" field on the "Accounts Receivable Sales Tax Report."

I am having a hard time reconciling the "Total Sales Tax" field to the "Sales Tax Collected" field on the "Accounts Receivable Sales Tax Report." Any idea how these calculations differ?  'Total Sales Tax" appears to reflect what is on the invoice (per tax rate table,etc.) - I am good with that one.  Its the "Calculated" field I am confused by.  Sometimes its greater, sometimes its less. Thanks