If you want that expression to yield a result of 635.9, you must insert a pair of parentheses inside the expression like this:
That yields a result of approximately 635.996 which I think is what you want. Your original expression yields a result of 638.25 exactly, which is not what you want. Also, be sure to be in STD display mode so that no digits are hidden.
Disclaimer: I don't work for HP. I'm just another happy HP calculator user.