Yes the testing conditions are very important, some people probably don't think about it when testing high power lasers. When measuring low power lasers make sure the air is still, windows closed and no heater or fan turned on, and don't have a light or lamp next to it or anything else giving off heat and don't hold the laser in your hand. I use two tripods a few inches apart and clamp the sensor on one and laser on the other and turn the laser on and stand back. This is what to do anyway to get better accuracy whatever power you're measuring.
Alan