I dont know for sure, but I do think the mileage requirements fluctuate based on inventory. I had recently booked a SLC-CUN in Y for 70k. Later on I found the same flight for 65k with one of the legs in F. I am guessing this was because Y was full and no one was in F.

I dont think it hurts to continue to watch the 'prices' which I do with all my flights.