If you see a null in anywhere in the value, you need not waste anytime in calculating the result. The answer will always be null. Hence the handy mnemonic title of this discourse. On the surface these types of numeric calculations may seem a bit non-intuitive, but reasonably palatable to the normal working of your brain. However the all-consuming nature of null values doesn't stop there.
Nulls behave differently.
Although the definition of null values is relatively simple, the implications for users of relational systems are not always intuitive. Unfortunately it also means that Ansi compatibility is not completely under the control of the database administrator. There are also special problems with stored procedures depending on your server version, how you compiled the sp and how it is being run. If you give it any other field and there is a null value in the field, count may miss it and your totals may be wrong. Since you can't perform any traditional mathematical operations on nulls and you can't count them, it should come as no surprise that you can't join on them either.