I'm a 1K and I got the 4/500 version. I've received miles basically every way you can (flights/hotel/car/dining/shopping/CC/prostitution) in the last six months, so I bet its based on your extracurricular mileage earning. If you already earn a lot of extracurricular miles, they don't have as much incentive to get you to rack up more. If you don't, then they're trying to drag you in...