[ozmerish]

When Amex launched the Blue card a while back, the interest rate was determined by your payment history - a sliding scale type of thing. Pay on time and you get the lowest rate, miss one or two and then another rate would apply. Keep missing dues dates and your rate goes in the tank.

Perhaps this is a variation on that theme? Spend more - they give you a better rate to carry a larger balance??

Just my 2c.

Cheers