Some unknown percentage of that is the Lizardman's Constant[0], some of them mean "not much", plenty of them use the map on their phone and don't think that's "using the Internet", probably a decent amount who use Facebook to look at pictures of their grandkids but figure they aren't using the Internet because they don't own a computer (if you told them a smartphone is a computer they would look at you funny). Some are religious plain folk and genuinely don't use the Internet, some are retirees who only have a land line (and genuinely don't use the Internet).
[0]: https://slatestarcodex.com/2013/04/12/noisy-poll-results-and...