Isn't it obvious? Your result is perfectly explained by integer division.
By the way, you would have much more luck if you multiply first:
int result = 100 * overload / order;
Better yet, work with floating point number all the time but round it to integer only when you show the result on screen:
double result = 100d * overload / order;
int uiResult = (int)System.Math.Round(result);
—SA