SPF should always hit?

[Robert Fitzpatrick]

Excuse me if this is too lame a question, but I have the SPF plugin 
enabled and it hits a lot. Should SPF_ something hit on every message if 
the domain has an SPF record in DNS?

Furthermore, a message found as Google phishing did not get a hit on a 
email address where the domain has SPF setup. Not sure if it would fail 
anyway if the envelope from is the culprit?

--
Robert