This is a quick, short but nifty trick in OS X. When I shifted from Windows / Linux to MAC, I used calculator the old way. Open the calculator first and then punch in the values to get results. Probably Spotlight thought a better way out and said why punch the arithmetic equations in some other utility when I can do that for you.
Yes you can do basic Arithmetic calculations within the spotlight itself. To do so -
Look at the image below to see how it works.