Why is users.get not returning information about 1000?

Question

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I call users.get with 1000 id. But in response, information about 900. What's the problem?
Checked here https://vk.com/dev/users.get

Answer 1

Options:

1. id are repeated
   
2. id and short name point to the same user
   
3. id are negative
   
4. id is not a number and not a short name