How can the sum formula possibly add 12 extra decimal places?

I am trying to get the actual total of a column of numbers using the sum formula, which I use all the time, and have never had this problem before.
There is not a single number in the column that has more than 2 decimal places, yet somehow the sum formula has added 12 extra decimal places into the answer... I've tried using the round formula, I've tried adding the numbers in a new workbook, I've tried retyping all the numbers, I've separated out the decimal values & confirmed that is no more than 2 digits in all numbers, but the answer still seems to be wrong.
Anyone know how I can fix this problem?