How is my credit limit determined?

Hi guys,

I know it's probably not an exact science (and this may be in the wrong place, so if so, please do close my thread or re-direct me!) but I wondered if any one could explain how the bank goes about setting the credit limit that you're offered?

I recently ordered a credit card in Natwest where I got a £5000 limit, however my wife, who shares a joint account with me, has been offered significantly less and her credit check came back good. Any one have any ideas?

Thanks in advance