I guess this has to do with Col Info > Format. In my JMP data, the first 2 columns were set to Fixed but the result column was set to Best. I think the calculation result actually took the values(contained 9-decimal point) from the 2 columns which was set to Fixed.
That data actually came from a test result so I guess I wasn't aware that it has so many decimal points :-)
Anyway, thanks a lot Jim for your response.