Why do I need to check for null fields when excluding fields with certain content?

Why do I need to check for null fields when excluding fields with certain content?

If I get all records and use
WHERE  "Lead Source"  != 'Webinar'
my report removes all records where Lead Source is Webinar or is null. It removes null fields. If I want to just get all records where Lead Source is not Webinar, I need to use this
WHERE ("Lead Source"  != 'Webinar'
 OR "Lead Source"  IS NULL)

Why is that? The logic in the first should be get all records where Lead Source is not Webinar. NULL does not equal Webinar. Why does the first example remove null fields? 
I feel like this can't be working right. Or I'm missing something.