Question on calories

I bought a box of instant Au gratin potatoes. It says calories as packaged 90 cal per serving. It also says 150 cal prepared. It says to add a half cup of milk and 2 tablespoons of butter. Do the numbers change depending on the product to use. For instance I have fat-free milk and Imperial margarine. Half a cup of milk is 45 cal 2 tablespoons of margarine is 140. That's a total of 185 cal. It says there's five servings per container so if you divide 185 into five you get 37 cal a serving. 90 cal as packaged +37 cal for the additional products comes up to 127 cal a serving. Five servings equals 610 cal, however the box is stating 5 prepared servings would equal 750 cal. How did I get that extra 140 cal, or is it just from other products being used such as whole milk or regular butter. Is my calculation correct at 610 cal for the whole box if I cook it the way I stated before. Thank you for your help