In your custom list which is having more than 500 records. When you query with REST API it returns only 100 records.

Eg: http://<domain>/<site url>/_api/web/lists/GetByTitle(‘ListName’)/items

Solution:
To override that you need to append $top=3000 or $top=5000

Eg: http://<domain>/<site url>/_api/web/lists/GetByTitle(‘ListName’)/items?$select=AuthorId&$top=3000

Here authorId is CreatedBy Column.

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