There is a normal fluctuation in service for peak travel periods versus the more dormant periods. It should also be noted that certain flights do not operate daily. Usually a timetable would have that information, but unfortunately due to the reservation system overhaul jetBlue's timetables are currently not viewable (btw its been a while already would be nice if they came back!). I noticed in October there were 4 daily NS flights as opposed to the 5 flights being offered through the summer. I am going to school in Seattle and trust me I am aware of this practice as after September SEA-JFK goes to once a day with two connection options, one in BOS and one in LBG (During the summer JFK-SEA has 2X Daily).
(Excuse the use of the word dormant, I mean off-peak)