Because one thing many emulators don't emulate (at least ZSNES does) is the brightness and tint controls that are on TVs and also the 1702. In the nesdev forum

Because one thing many emulators don't emulate (at least ZSNES does) is the brightness and tint controls that are on TVs and also the 1702. In the nesdev forums there was some interesting discussion threads about trying to get an accurate NES palette. Someone digitized each color from a real NES using a capture card, but the discussion talked about how people's TVs of the day probably weren't scientifically calibrated; everyone had different tint and brightness settings on their TVs. That being said something was always off about the red color in every c64 emulator, it never seemed that "grey" on any TV or monitor I ever saw.