Because your values in that cell isn't a number--it's text that's masquerading
as a number.

One way to fix a bunch of cells is:
select an empty cell
edit|copy
select the offending range
edit|Paste special|Add

and reformat it the way you want.

dm wrote:

When I format a column to be zip + 4 or phone numbers, I must first
double-click the cell(s) then press enter for the change to show up. Why?

--

Dave Peterson