That’s probably when their IP switches as they move from one page to another on your site. So, for the 10 seemingly different visitors I mentioned above, the first one might show a search engine or something else as a referrer, and the other 9 (that are really the same person) will show up as being on your site already. Their IP addresses change on the fly.
I’m not sure how often they switch… it’s not a page to page thing, but it’s pretty frequent.