The portrait lumia calculator is a basic calculator. It does equations one step at a time. e.g 2 + 3 = 5, 5 x 4 = 20.

The scientific (landscape) calculator reads it as 2 + (3 * 4) = 14.

It works out the same with the windows calculator and normal calculators. Try it for yourself, it's not just a flaw of the lumia 800.