I haven't got a clue, to be honest. I've only been collecting credit cards for 2-3 years now, and my credit limit is already up to well over 13k - I only make 24k per year, and definitely don't lie about anything on my applications. I'm pretty sure I could keep applying for credit cards and having them handed to me. Discover even offered me a personal loan up to $25,000 - you know, for debt consolidation. Not that I have any debt.
I think I get like, 5-7 offers a week from various companies? They're probably the only thing that keeps the USPS in business anymore.
Good thing - or bad thing? - for those credit card companies that I don't suck at money. : )