A number of years ago I made a data table and inserted some simple formulas
in two or three end columns. Each year I simply add another row and copy the
one above. This time I noticed that the cells I copied to the empty row
carried the results of the calculation above as though the underlying
formula (which should have calculated a zero because all the feed data cells
are empty) were working on the line above. I've check and double check but
can't find why this copied as it did. It's as if I copied text rather than a
formula. When I rewrite the formula, all is well; I get my zeros. There must
be something very basic I've forgotten over the years. Any clues?

Example of issue:

End column O on row 40 contains: =SUM(C40:N40) as underlying formula with
results of "1074.16."

When that cell is copied to the cell immediately below on row 41, results
appear as "1074.16" with the same underlying formula. The program made the
proper adjustment for the new row (=SUM(C41:N41) but the results should be
"0" since C41 through N41 are empty.