There will also be measureable temperature gradients throughout the chamber, probably a degree or two warmer near the ceiling compared to the floor, walls will likely be a bit cooler than the center, etc. There's no way the entire volume of a typical environmental chamber could be kept within 0.1°C of a set point. Maybe a small region could be kept close to that with metrology grade RTD's, instrumentation, and well-tuned proportional control but that would get prohibitively expensive.