I have a friend who just tried to book a trip using Delta SkyMiles. According to the Delta SkyMiles Award Chart, the ticket should have 'cost' her (one way) 12,500, 20,000, or 30,000 miles. At first I thought this was nothing new.
In the past, different mileage levels typically referred to flying at peak or off peak times. My husband and I saved a bundle of miles on air for our Holy Land trip because we were flying to Europe off peak. We flew American.
On the Delta chart site I couldn't find anything that really clarified what the different mileage levels meant. Apparently my friend didn't either until she went to 'buy' her ticket and found different mileage levels FOR THE SAME DAY! Not only that, but she found the mileage levels changing from day to day.
I went to Delta's site to 'buy' a SkyMiles ticket. Picking an arbitrary date for Florida travel, going and coming in January, I found flights, for the same day, ranging in cost from 12,500 to 20,000. For instance, returning from Florida early in the morning cost 12,500 but if I wanted to fly out around noon, the cost was 20,000. This is the first time I have seen multiple mileage 'costs' on the same day for the same economy seats.
American doesn't appear to have changed from the on peak/off peak I am familiar with. I was playing around with American mileage seats for a potential future trip. As expected, all available economy seats for the same day had the same mileage cost. Interestingly though I found some seats blocked out as Premium. Since I was just playing around, at this time I have no idea what the additional cost of a Premium (aisle) seat would cost, in either dollars or miles.