I think it is based more on a particular stay pattern. Basically, those that have stayed more within the past year or so, will get the higher end offers. Those who have stayed a moderate amount over the past year or so will get the middle offers. Those who have stayed fewer nights will get the low end offers. This will, generally, translate into exactly what you suggest (Plats get the high, Golds get the middle, Silvers get the low.)