There were also instances where deceased people were misrepresented as still living so relatives could collect on their pension. When the government began cracking down and the numbers corrected, the reported population of extreme elderly shrank. Obviously it isn’t the only reason, but it may have some impact.

https://abcnews.go.com/amp/International/scandal-missing-100-year-olds-graying-japan/story?id=11393615