Answer:
Lower-income people can have a large vocabulary as well, because of a variety of circumstances. To list a few:
The low-income person may have a job that includes a constant contact with highly-educated people.
The low-income person may himself be a well-educated person who holds a job that does not pay a lot, for example: librarian, or social worker.
The low-income person may live in an area with a great degree of demographic diversity, and even by fluent in several languages because of that. This is common in merchants, who may own a small business that only pays enough to survive, but who have a great degree of contact with many sorts of people.
