UDF returns the same value everywhere

I am trying to code in moving average in vba but the following returns the same value everywhere.

Function trial1(a As Integer) As Variant
    Application.Volatile
    Dim rng As Range
    Set rng = Range(Cells(ActiveCell.Row, 2), Cells(ActiveCell.Row - a + 1, 2))
    trial1 = (Application.Sum(rng)) * (1 / a)
End Function