I have 7 rows of data in one column, the SUM of which adds up to Zero,
as expected, except it doesn't. In the SUM cell formatting to 13
decimal place shows as 0000etc13. I've even retyped the 7 rows of data
(max of 2 decimal places) and my result is still the same. I even
pasted-special values ROUND after that and same result

How can that be?