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It’s not too difficult a question. I’ll try to explain without mentioning any spoilers. The translator basically rearranged the sequence of the story. There is a flashback set in the Cultural Revolution period. The flashback is in the middle of the novel in the Chinese version, whereas the English translation starts with the flashback.
According to author Liu Cixin, the story was supposed to start with flashback. However, since the original novel was first serialised in Chinese magazine Science Fiction World, and the publisher chose to move the flashback to the later part of the novel to avoid attracting attentions of censors.
So when they were working on the English translation, the author was able to use the sequence he intended, with the flashback as the opening chapters.