There’s no reason to expect that the sum of the balances remaining would be equal to the initial amount. If you spend in 1$ increments, the balances would be 49, 48, 47,… which sum to much larger than 50. If you spend 49$ followed by 1$, right away, the balances sum to 1$, which is much less than 50. This particular example has been “cooked” to give a value of 51. You can cook up another example to give whatever value you want.