Just finished writing up some code to grab Temp / Humidity values from Measurement Specialties HTU21D (and probably Sensiron SHT21) sensors.
Temperature seems pretty close to other measurement gear I have. It's one of those tiny breakout boards from Aliexpress (similar to the Adafruit/Sparkfun boards).
I have a feeling the onboard voltage regulator is putting out a tiny bit of heat making it read a bit high. Won't need an LDO on my final design PCB as it has 3.3V for the ESP8266 anyway. Datasheet also has some good tips on using routing slots to provide some thermal barrier between the IC and the rest of the board.