I just spent an hour trying to figure out why a SUMPRODUCT function gave the wrong value. I was sure that it was due to my not using the function correctly.
So I created a new book that contained only the function and a bit of data intending to post it here to get assistance with the function.
Lo and behold, when I put the function in the new book it worked fine.
So I tried eliminating everything 1 by 1 to make the 2 books look just like each other.
At the very end I decided to do a Paste Special->Column Widths and suddenly the “good” sheet went “bad”.
It seems that Excel rounds numbers when the column is too narrow to show all the decimal places.
I have tried many searches of help and the web but have not found this documented anywhere.
Beware!