Couldn't it be there from someone who knows you? I was under the impression that Facebook grabbed people's contacts list on mobile apps.
Another option would be that someone else has that number listed for their account. Has Facebook always required confirmation that a number is valid? I saw one my friends' numbers in the data except the account had a different name.
If I give someone's social security number to a company that doesn't mean they can publish it on the front page just because the person it belongs to didn't hand it over.