Why 10% lower? Because the led battery was shielding the radiation detector.
From Kyodo News (11/7/2012):
Radiation levels displayed 10% lower, 675 monitoring posts to be repaired
The Nuclear Disaster Response Headquarters of the Japanese national government announced on November 7 that the radiation monitoring posts set up in Fukushima and the neighboring prefectures have been showing air dose levels about 10% lower [than the actual levels], and that the repair work will be done on 675 of them.
The metal box containing the battery is placed next to the radiation detector inside a monitoring post, blocking the radiation.
Residents and municipalities [in these prefectures] have pointed out that the levels displayed by the monitors are different from the dose levels [that they measure], so the government will modify the location of the equipment and try to have a more accurate reading.
In response to the Fukushima I Nuclear Power Plant accident, the national government has set up the radiation monitoring posts in public buildings and tourism destinations, and the monitoring results have been available on the homepage of the Ministry of Education and Science since this April.
Jiji Tsushin says it was the lead storage batteries right next to the monitoring post shielding radiation from the ground, and there are 545 monitoring posts in Fukushima, and 130 in neighboring Miyagi and Yamagata and other prefectures (the article doesn't say where).
10% lower. And that's on top of the reduction of radiation by replacing the soil/concrete around the monitoring posts, and after a sizeable decrease in cesium-134 (half life 2 years) in the environment.
The original vendor contracted to build these monitoring posts (Alpha Tsushin) was summarily dismissed in November last year after their monitoring posts displayed the radiation levels "too high" for the comfort of the Ministry of Education. The current monitoring posts are supplied by Fuji Electric, one of the top four heavy electric companies including Hitachi, Toshiba, and Mitsubishi, and Hitachi-Aloka Medical.
These four companies do a healthy business contracting from the national and local governments on large and small public projects. Always go extra miles to please the price-insensitive customers.
Jiji Tsushin's article also says:
Ministry of Education says it did not expect the shielding effect of the batteries.
Yeah right. I can almost hear the conversation that may have taken place between an official at the Ministry of Education and the favored vendors like Fuji Electric and Hitachi:
Official: "We have a problem. Alpha Tsushin's monitoring posts are scaring the residents. The numbers are just too high, and we don't think it's right to scare people who have suffered so much (and whom we need to stay put where they are). So we're going to fire the vendor. What can you offer?"
Fuji, Hitachi: "Well, we could build monitoring posts that display more reasonable numbers."
Official: "Do it. I don't care how you do it, and don't tell us how you do it."