What kind of influence does interface exhibit on computational result in Creo TAE?

I set all the interfaces property to "Middle", but the results is still different between TAE automatic cacultation and MathCAD manual caculation, what kind of influence does interface makes to the caculation process?

Would you mind tell me the fomula?

(In this picture, you can see the results is different between TAE automatic cacultation and MathCAD manual caculation)