Range(add1, add2).autofill Works

Range(add1,col1 & Range(col1 & Rows.Count) Means
Range("B2", "21") "1" is the Row that Range("21,048,576").End(xlUP).Row will return. Unless you accidentally put some value in the column Cells("21,048,576") happens to fall in.

IOW, "Range(add1,col1 & Range(col1 & Rows.Count).End(xlUp).Row)" is GIGO And I don't know what exactly you are trying to do.

Run this Code 
Range("A1").Select
MsgBox "Message #1: The Active Cell is " & ActiveCell.Address 
col0 = ActiveCell.Column
ActiveCell.Offset(0, 1).Select
MsgBox "Message #2: The Active Cell is " & ActiveCell.Address 

add1 = ActiveCell.address
col1 = ActiveCell.Column

ActiveCell.Value = 1
ActiveCell.Offset(1, 0).Select
MsgBox "Message #3: The Active Cell is " & ActiveCell.Address 

add2 = ActiveCell.address
ActiveCell.Value = 2

MsgBox "Message #4: The wierd address is " & Range(add1,col1 & Range(col1 & Rows.Count).End(xlUp).Row).Address