Answer: 

That seems normal, taking into account that your heat exchanger is requested to use the internal diameter (see general geometry dialog)

The input for the shell is 8 inch NPS

Normally an 8 inch NPS pipe has an external diameter of 8.625 in. Since the model is defined with internal diameter as shown on the General geometry dialog above, the software starts with an internal diameter equal to the 8in NPS value.

If your wall thickness is equal to 0.25 in
8.625 + 2 x 0.25 = 9.125 in

The problem does not occur if you choose in the general geometry dialog the option "Nominal" and select the pipe size.