The declarations, that draw on the annual Japan Atomic Energy Commission reports "The Current Situation of Plutonium Management in Japan" reported that Japan had 4.5 tonnes of plutonium in unirradiated MOX fuel. According to the Current Situation report, this number included 959 kg of fresh MOX fuel at power plants. This number, however, did not take into account the 640 kg of fresh MOX fuel that was loaded in Genkai #3 reactor on March 9-12, 2011. Since the reactor never went operational, the fuel was still unirradiated, but was not accounted for in any of the categories of the Current Situation report (or INFCIRC/549). It was removed from the reactor in March 2013 and may be again reported in Japan's 2013 INFCIRC/549 declaration.
The discrepancy was reported by Masafumi Takubo, a member of the International Panel on Fissile Materials, who reported it at Kakujoho, a nuclear information website.