Did you not read what he said? You are just looking at whether it goes up or down. Being perfectly accurate is not necessary.

Its like if you had a weight scale. If it told you, tomorrow that you gained 15.3 lbs, and you repeated... and it said 13.9 lbs... 17.6 lbs, so on. It doesnt matter the exactness. The point is, your weight went up a lot in one day. That is good enough to make decisions on. Not for scientific studies.