Excel 2007 : Unwanted Conditional Formatting Rules

Apologies if this was addressed elsewhere, but I didn't find anything on a search.

I have a worksheet with several columns of data. I have applied the standard gradient formatting rule to each of those columns, with higher values in the column in green and lower values in the column in red. As I add new rows of data, I periodically re-sort the rows by one of the columns, sometimes ascending according to one of the columns, and sometimes descending according to another, depending on the trends I want to see. Sometimes after I do a re-sort, I notice that some of the colors change, and if I click on "Manage Rules" for one of the conditionally formatted columns, I notice that an additional rule has been added that applies to one cell in that column. Sometimes that does not happen, though, and I have been unable to determine the circumstances under which the additional rule magically appears.

If anyone has any thoughts, they would be much appreciated.

Sorry...typo: I have NOT been able to determine the circumstances under which the unwanted rules are added.