Excel 2003 Win XP Home

I use a simple worksheet which consist of five columns into which I add four
figures of numeric data across a new successive row each day. The fifth
column includes a sum formula to sum the entries on that row's four columns.

Here's the mystery. When I enter the fourth of the data figures on a row,
the sum magically appears in the fifth column. I've checked and the sum
cell is blank prior to my entering the data figures. Somehow, Excel has
figured out that the new row should contain the same formula as the cell
immediately above.

How does this happen?

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PT