Under normal circumstances, "not following back" page shouldn't show the accounts you are currently following.
We are directly getting the data from the Twitter APIs and listing them as is. On the other hand, Twitter APIs might send wrong data rarely. Twitter indicates this small mistake on their rules, but it wouldn't be a significant percentage. (It is also a common issue among the Twitter community here: https://twittercommunity.com/t/discrepancy-in-followers-count/80018/20)