Why do fortune tellers calculate things accurately in the past, but not particularly accurately in the future?

It's normal, one is the innate internal environment, and the other is the acquired external environment. The latter is constantly changing, and any new person or environment may change a person, which is unpredictable. Some people's knowledge and wisdom will also change the innate eight characters, which cannot be accurately said. The past is easy to judge, because you and your surroundings are predictable. So what you said is normal.