>the summation works fine This is only true if that summation is not used or compared anywhere else which isn't usually the case. For example if in sql you are

>the summation works fine This is only true if that summation is not used or compared anywhere else which isn't usually the case. For example if in sql you are reporting the SUM(column_A)/ SUM(column_B) and one column can be null without the other one being null you're likely reporting the wrong number