I don't know where LinkedIn got my contacts from but I suspect I must have missed a setting when I briefly installed the mobile app a few years ago. Some of the email addresses they have are out of date so that adds weight to my theory.
When I signed up I saw a bunch of people I had as contacts on a gmail account which I hardly ever use. I had NOT used said gmail account when signing up for LinkedIn (and no LinkedIn app etc.). They seemed to have somehow associated me with contacts for an email address I had told them nothing about by piecing together the contact lists of the small number of contacts I added on LinkedIn and linked the name in their address books to my unnamed gmail account. I simply don't trust them anymore.