Why do conditional formats appear by themselves in Excel 2003?
 

I have an Excel 2003 spreadsheet which sometimes (but not always) chooses to
copy a conditional format into cells when I type a value into them. When it
is in that mood, I have tried looking at the cell format, which is "no format
set", typing the value in, then looking again - the full conditional
formatting is now set. There are no cells in teh column with conditional
formatting, but there are some elsewhere on the sheet.

Why?