This is probably it (excerpt from the help):

Turn on or off extended formats and formulas
When you turn on extended formats and formulas, Microsoft Excel
automatically formats new data that you type at the end of a list to
match the preceding rows and automatically copies formulas that repeat
in every row. To be extended, formats and formulas must appear in at
least three of the five list rows preceding the new row.

On the Tools menu, click Options, and then click the Edit tab.


Do one of the following:
To automatically format new items that you add to the end of a list to
match the format of the rest of the list, select the Extend list
formats and formulas check box.

To prevent automatic formatting, clear the check box.

Hans