Impressions measure exposure, not people. They grow when the same followers see a post again, which is why they are always at least as high as reach.
Many platforms now call them views. A high number of impressions with a low reach means the same people keep seeing the post. A reach close to impressions means it keeps finding new people.
Example
A post with a reach of 800 and 2,000 impressions was seen two and a half times per person on average.