If the cell is formatted as General (the default) then it will
automatically display as scientific notation if the column is too
narrow. It will revert to normal notation if the column is autosized.
If the column is formatted to Number and the column is too narrow, the
Excel will fill the cell with ####. Again, the number will look normal
when the column width is widened enough.

So if you want the display to look correct, have your macro do a
Columns("A:A").EntireColumn.AutoFit statement after concatenating all
the values.