The one per month rate that I'm comparing it to is the previous months of the same list, and also includes only unnatural deaths.
It is difficult to die unnaturally in Japan, and given the small number of US Citizens living there, it is certainly possible to have long periods of no unnatural death.
By that reasoning there shouldn't be a death per month in the previous years. But there was. Going from that to nothing is a drastic change and rather improbable.
That's possible, but it's worth pointing out that only unnatural deaths are cataloged. This does not include deaths by a natural cause.