The same table on the Wiki page says that the actual count was 289 murders in 2022. Japan had a population of over 126 million in 2020. If we used your formula, that would mean there were over 289,000 murders in Japan between 2021 and 2022. See below:
230 per 100,000 = 126,146,099 x 230 ÷ 100,000 = 290,136 murders total.
Whereas, 0.233 per 100,000 = 126,146,099 x 0.233 ÷ 100,000 = 294 murders total.
2+2=4, simple maths. But 290k murders per year would explain the population drop, lol.
As the other guy mentioned, 0.23% per 100,000 doesn't make sense. That's like saying 0.23 murders per 100 (cent) per 100,000. One of those per Xs is redundant.
The Wikipedia page linked says Japan has a murder rate of 0.233 per 100,000 residents. If this made sense as a percent, it would mean a percentage of 0.000233%. But this statistic wouldnt work as a since a "percentage" implies a fraction of a whole, for example 50% of a population is born male.
The original statistic under discussion says that there are "X number of murder cases per 100,000 people living in country", rather than "Y% of the country's population is murder victims", which wouldn't make sense anyway since the dead aren't part of the population; i.e. you wouldn't see a statistic like "20% of the population is dead" but rather "20% of the population died", and "90% of the deaths were by murder" rather than "18% of the population is murder victims".
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Edit: formatting
Edit: 0.233 murders per 100,000 residents (in 2022) = 126,146,099 (pop of Japan as per the 2020 census) x 0.233 ÷ 100,000 = 294 murders total. The wiki page says the actual count is 289, so the error is probably due the drop in population since the 2020 census.