Except that's not quite true. Each time you pick one up and move it around, you may lose a little sand. "Sand" is only lost when the number has a huge (>53) number of digits in binary.

This usually happens because of the use of numbers that are periodic in binary; they have an infinite number of digits. The OP used 425/10000 and 25/10000 which are both periodic in binary (like 1/3 is in decimal).