What is showing on the Nest's screen? Does the Nest know that it's 75 now, and that it's only supposed to heat to 68? Does the Nest show that it's currently calling for heat even though the room temperature is above the set temperature? Or is your heat running even though the Nest isn't calling for it to run? The question is whether there's an issue with the Nest itself, or how it's programmed, or how it's wired, or your heating system itself.