Something I didn't (and still don't) know about SQL

Take this simple Database Table:

Field1

(null)

abc

xyz

Here's a test for you. What will the following query return?

select * from table1 where field1 like 'a%'

If you answered abc you are correct - congratulations!
Ok now here is the same query slightly modified - what will it return:

select * from table1 where not (field1 like 'a%')

Surprisingly (to me) it doesn't return (null) and xyz, it only returns xyz. If anyone knows why can you please explain in the comments?