十进制与双数学精度

Question

我注意到数学运算有时对于小数而言不如对双打更准确。例如，当我运行此代码时：

    Dim a As Decimal = 1
    Dim b As Decimal = 3
    Dim c As Decimal = (a / b) * b

    Dim x As Double = 1
    Dim y As Double = 3
    Dim z As Double = (x / y) * y

    Console.WriteLine(c)
    Console.WriteLine(a = c)
    Console.WriteLine(z)
    Console.WriteLine(x = z)

输出：

0.9999999999999999999999999999
False
1
True

如您所见，Decimals给出浮点问题（0.999999），双打给出确切的数字（1）。

如果小数应该比双打更准确，为什么会这样呢？