Wrong coding or RANDBETWEEN is wrong?

I've try to solve this question: http://www.excelforum.com/excel-prog...32#post3340932

Now, first I've used solution with RANDBETWEEN function:

Sub fill_random_x()
Dim x As Integer, y As Integer
Dim rng As Range
Set rng = Sheets("Sheet1").Range("A1:C6")
   
'For m = 1 To 100000
rng.ClearContents
   
Do While Application.WorksheetFunction.CountIf(rng, "x") < 6
    x = Application.WorksheetFunction.RandBetween(1, rng.Rows.Count)
    y = Application.WorksheetFunction.RandBetween(1, rng.Columns.Count)
    Cells(x, y) = "x"
    'Cells(x, y).Offset(, 5) = Cells(x, y).Offset(, 5) + 1
Loop
   
'Next m
   
End Sub

However, I got bad distribution:

4,60%	5,63%	6,42%
7,03%	5,70%	4,05%
6,64%	4,45%	5,55%
4,02%	6,96%	5,65%
5,53%	6,71%	4,50%
5,65%	4,03%	6,88%

With this approach it looks fine:

Sub fill_random_x_v2()
Dim x As Integer, y As Integer
Dim rng As Range
Set rng = Sheets("Sheet1").Range("A1:C6")
   
rng.ClearContents
   
Do While Application.WorksheetFunction.CountIf(rng, "x") < 6
    x = Int(1 + Rnd() * rng.Rows.Count)
    y = Int(1 + Rnd() * rng.Columns.Count)
    Cells(x, y) = "x"
Loop
End Sub

And result is:

5,53%	5,58%	5,59%
5,53%	5,59%	5,53%
5,56%	5,57%	5,54%
5,51%	5,55%	5,53%
5,57%	5,57%	5,58%
5,55%	5,58%	5,54%

Is there a my fault or really RANDBETWEEN doesn't work as expected?