Pre-approved means nothing

A few weeks ago I got another "You're pre-approved for this credit card!" letter in the mail. I was actually interested in this one because it earned points toward free groceries so I applied for it. Well today I get a letter back saying that my request has been unapproved. Now I have 2 questions about that. #1: Why would I be unapproved? I have great credit! #2: The original letter said I was pre-approved. But apparently I wasn't. So basically they were lying? Gee nice.