Detail Tax Lines - why are they not consistent

Hi

When looking at Detail Tax Lines on an invoice (Receivables>Transactions>Transactions>Tax (B) - Brings up Detail Tax Lines), how does AR or eBTax assign the detail tax line information?  For example, we calculate State, County and City tax on each invoice line.  Thus, we have 3 Detail tax lines for each invoice line.  However, Detail Line 1 can sometimes be State tax, sometimes be County tax or sometimes be City tax.  It's not consistent.  Can anyone explain the significance of the assignment of tax type to detail lines?  I'm asking because we have had trouble with Tax Adjustments being randomly assigned