Some of it comes from importing words from different parts of China or at different times in history. From my (very) limited experience, Japanese pronunciations of kanji are much closer to Cantonese or Hokkien than Mandarin.

There is a long list of exceptions for Chinese: https://blog.tutorabcchinese.com/mandarin-chinese-learning-t...

Kanji will have at least two pronunciations (one called a Chinese pronunciation that often sounds little like modern mandarin, like 电 being Dian in Chinese and Den in Japanese).

A sibling comment shared a short list of common characters with multiple pronunciations. If we include variants where the difference is only tone, then the list of common-use characters with multiple pronunciations gets pretty long.

This list is also changes on a regular basis. The following news article details updates to pronunciation guidance promulgated by the Taiwan Ministry of Education at the end of 2022: https://www.ftvnews.com.tw/news/detail/2022326L08M1

It is my understanding that these new guidelines result in mandated changes to educational curricula as well as standardised testing; they are not wholly advisory. It is also my understanding that these changes can be mildly controversial, because even when the changes are made for the sake of simplification, they lead to discrepancies between the educational standard and common spoken language.