Change the survey machine, then the radiation levels may double, as they did in Misato City in Saitama.
A sort of "Shadenfreude" for citizens and particularly parents whose own radiation measurements were ridiculed and discredited by the authorities as "amateurish", "prone to mistakes", "using cheap and inaccurate Chinese-made survey meters", etc., etc..
Misato City in Saitama Prefecture is in the "corridor" of the radioactive plume that reached from the north and continued on to eastern part of Tokyo and western part of Chiba.
(From the radiation contour map (ver.4) by Professor Yukio Hayakawa of Gunma University; I marked the location of Misato City.)
After concerned parents finally persuaded them to measure the radiation levels in schools within the city, the city government started to measure, using Hitachi Aloka TCS-161 which wasn't working too well. Then the city government upgraded the survey machine to Hitachi Aloka TCS-172B on September 6.
Lo and behold! The radiation levels in school yards jumped; at several schools they more than doubled.
According to Misato City website, here's the survey on August 29 on the left, measuring radiation in the middle of the school yards using the old machine. At these schools, school yards are dirt. The survey on September 12, using the new machine is on the right. The numbers are in microsievert/hour:
The explanation by the city is simply "there is a change in numbers because we upgraded the survey machine."
One interesting thing about the new survey is that the radiation at 50 centimeters off the ground is the same as or higher than 5 centimeters off the ground, across the board, and there is not much drop-off at 1 meter off the ground.
At 0.31 microsievert/hour, if you stand in the middle of the school yard for 24 hours a day for one year, you would get 2.7 millisieverts of external radiation exposure. Of course you don't stand in the middle of the school yard 24/7, but if anything, the middle of a school yard may have lower radiation level than bushes, side drains, roads, and homes.