Going to guess at this;
JFK is probably <1 mile from Jamaica Bay and beyond that obviously has a longer fetch seabreeze off of the larger and much deeper Atlantic Ocean.
HVN is further from the coast and can have a fetch of ~20 miles, Long Island Sound is not as deep. I believe in the area of 65 feet for the central area. Maybe the Sound Breeze takes longer to develop and thus HVN can have a higher maxima before the onset.
Other than that, I got nuttin'.