Hi: My refund is not per tax return computation by TurboTax. I don't owe any back taxes. Haven't received any explanation from IRS. How to find out why the discrepency?

My refund not per Turbo Tax computation. I don't owe any back taxes. 
How to find out why there is a difference between Turbotax refund computation as submitted via Tax return and what IRS has refunded me? 
I haven't received any notice or explanation letter. Please advise how to proceed.