arguably, the feature of the application, and the egg of Columbus, is to utilize the additional steering pad of S60 phones with the arithmetic operations (not having tried the application myself, due to the incapability to find reason of the price vs. utility) added with the numeric keyboard found on mobile phones. so at such, I don't see the need for displaying addition buttons with numbers, but, as you say, most phones have means to enter arithmetic functions as plus and minus etc e.g. with the "*" button. but however you enter the arithmetic, f.ex. 2 times pushing the "*", the speed of the result will _still_ be determined how fast you push the buttons, IMHO.
I dont know how it handles 131070/2, or 850*77.1, but actually I bet it outperforms the precision of excel 2007 in those equations, even not based on how fast you push the buttons.