One of the other issues I have pointed out in the past. The 757’s are being retired and fewer are available. There were a bunch of A320’s operating out of STT however the FAA decided we are all getting fatter and bringing on more carryon baggage so they raised the assumed passenger weights. This appears to have pushed most of the A320’s out of STT. It might be that STT-BOS can’t be flown with a A320 economically because of load restrictions. Now you see a lot more of the A319’s and short 737’s.