A funny error

Dear All,

I have written a finite element code in C++. The results have generally high accuracy. But when I change the order of evaluating r and theta of the polar coordinates in terms of x and y in the sub-program of computing the shape functions, then the results are changed about 10^-9 of the initial results. The change is negligible but I wonder why this happens? I had initially supposed it may be related to some memory linkage in the program but since now I have not been successful to find any. Is there any one to have an idea about this funny error???