关于VB计算器的问题

为什么当2个有小数的数字计算后，结果会显示出带有10多个小数的数字？比方说3.2-1.2= 1.99999995231628
为什么会出现这种情况？以下是我所有的代码：

Dim Runsign As Integer '储存运算符号
Dim Num1, Num2 As Single
Dim StrNum1, StrNum2 As String
Dim FirstNum, SignFlag, PointFlag As Boolean

Private Sub Command1_Click(Index As Integer)
Select Case Index

Case 0 To 9
If FirstNum Then
StrNum1 = CStr(Index)
FirstNum = False
Else
StrNum1 = StrNum1 + CStr(Index)
End If
TxtShow.Text = StrNum1

Case 10
If PointFlag Then '假如没有小数点
If FirstNum Then '如果是第一个字符
StrNum1 = "0."
FirstNum = False
PointFlag = False
Else
StrNum1 = StrNum1 + "."
PointFlag = False
End If
Else '已有小数点
Exit Sub '退出单击事件
End If
TxtShow.Text = StrNum1

Case 12 To 15
FirstNum = True
PointFlag = True