Perhaps a CO person can help. This is the same problem we see over at EWR on CO. What is happening is that DL is protecting its international gateway at JFK for connecting passengers coming off their international flights wanting to connect to a DL flight to MCO. If you sell out the plane with cheap fares, not only have you lost your revenue from selling more expensive fares, but you have also limited your international connecting passengers from getting to MCO from JFK. In order to prevent that, DL simply sells Y fares, thereby limiting the sold seats to only high fare passengers (the ones you alway want to sell tickets to). This leaves plenty of inventory to the connecting international passengers.

As the time gets closer to departure, if excess inventory exists, then DL can open lower fare catagories and sell discounted tickets, which is why you may see cheap seats open 2-5 days before departure. However, I can say at EWR, it is very rare to find inventory especially to other hubs such as CLE, ORD and IAH.

It is also why LGA is cheaper. There are no inbound international flights to speak of and the need to preserve inventory for them does not exist. You can sell discounted seats from the getgo. This is also the case with CO at LGA. My flights to CLE from LGA are much cheaper than EWR to CLE. Hope this helps somewhat

Radonc1