Cause of difference of tax amounts using api and auto invoice program

I have created 2 AR invoices 1 from api and 1 from auto invoice program after interfaced sales order lines. There is a difference of tax amount in both invoices however the order value, unit selling price and ordered quantity are exactly same. My environment is 12.1.3 with database 11.1.0.7.0 release 11g. What may be the cause of this difference. I was not expecting any difference because both functionalities of invoice creation are seeded. Therefore they should create identical invoices by all means.