MLX90614 BAA Thermal Sensor

We are using MLX90614 BAA IR sensor to sense the body temperature in a hobby project we are currently working on. The temperature is changing dramatically even in 2-3 cm range from the sensor, and in these ranges, we observe approximately -3 Celcius degree error. From the calculations made on Field of View information given in the datasheet, we expect the sensor to measure the temperature accurately in this range. Are we supposed to set some configuration of the sensor before using it?

I am not able to answer this question. Is there anyone out there who is able answer this?