I think your numbers are off. Just playing with Google Flights, I can book up to 3 in the P bucket with the 7:30AM flight.

If you search on the 7:30AM as standalone, it shows only 3 bookable in G bucket. After that, it goes to the A inventory bucket. 8:45AM has more G bucket available. So the limiting factor is likely really the 7:30AM flight to PHL. Probably more popular for business flyers than the 8:45AM flight and DL is likely holding inventory back to sell more of that flight as a standalone rather than as a connecting itin.