I always figured that was just elevational effects mainly. PIT is at 1201', while SYR is 420' and BTV at 330'. There's usually not nearly as much of a north-south gradient in the summertime - in fact, many times with a big summertime ridge, there's warmer air aloft spreading in from Canada than there is further south. The moist adiabatic lapse rate is 3.5F per 1000 feet and dry adiabatic lapse rate is 5.4F per 1000 feet, but superadiabatic lapse rates can occur. Plus because of SYR and BTV's location, they get downsloping from most wind directions. PIT would only have downsloping from the east, otherwise generally a mild upslope flow (1200' being one of the higher points, which makes sense because it's an airport - there are a few 1300-1400' spots).
If you look at last year, PIT had 5 90+ days, while SYR had 18 and BTV 14. However, BGM at 1627' only had 2 such days. I imagine if records were still kept downtown at say 800 or 850 feet, there would be more. The hot Syracuse temperatures are always a point of contention with the locals here, but elevation effects alone suggest it should be 4-6 degrees cooler in an atmosphere with adiabatic lapse rates than nearby Binghamton. I would imagine subtle downsloping effects and a more urbanized environment could even enhance those numbers somewhat. Obviously, in the winter, these trends don't hold. Temperature inversions are common, and lapse rates lower even in the absence of a temperature inversion.