中英文在一起时，为什么把中文辨别为1个字节,中文是2个字节呀

Public Function LeftGBB(ByVal str As String, ByVal num As Integer) As String

    If IsNull(str) Then
        LeftGBB = vbNullString
        Exit Function
    End If

    Dim strinfirst As String

    strinfirst = StrConv(LeftB(StrConv(str, vbFromUnicode), num), vbUnicode)

    '去掉尾部多余

    If Asc(Right(strinfirst, 1)) = 0 Then
        strfirst = Left(strinfirst, Len(strinfirst) - 1)
    End If

    LeftGBB = strinfirst

End Function