ok guys,
let me try to explain this in geometric language.
the slope of the red triangle=p/b= 3/8= 0.375 and
the slope of the green triangle=p/b= 2/5= 0.4
which shows that the hypotenuse of the whole triangle is not a straight line 
(the slopes of individual hypotenuses being not equal)
now, if you place the triangle with greater slope in the bottom and the one with 
smaller slope on the top, it happens to occupy more space beacause of its 
convexity.....notice the small squares in the corner are shaded more in the second 
triangle.
the difference happens to be equal to 1 square unit.