We discovered that (and this is also confirmed by Netsuite support) single order processing in UI differs from bulk processing when it comes to calculating taxes. Bulk processing uses a different method of calculation or rounding sometimes end up being different than what Avatax reports.

Example;

Sales Order shows tax amount $12.32. Avatax tab also shows the same amount (i.e. Avatax service returned the value of $12.32 and Netsuite recorded it correctly). When this Order is bulk processed (Invoiced/Cash Sale'd) with others, it creates the transaction record with a tax of $12.31. When I look at the Avatax tab of this record, it was calculated as $12.32, and not $12.31. Meaning, Netsuite did not record this number, instead it used some shortcut method or something to calculate the tax.