I have been seeing the same thing. It is not a matter of flights selling, but a flight will jump to another fare bucket when booking. I tried on travelocity with the same results.

I am trying to book a trip to MIA in June (about 10 weeks out). I do mostly all of my travel booking at short notice (1-3 weeks usually) so I was wondering if some of the lower fares (LUT) were ever held back until closer to the date????

I am sure this has been discussed before, but I was unsure what to search on. I looked at the fare buckets, and the lower fares are just desimated.

Am I trying to book too far out and more, lower fare seats may come available closer to the date? Or is Delta just cutting so many flights that it is becoming that much more competitive to find cheap fares with this much notice?