The microblogging network Twitter has not included a new advertising channel in the list of followers you have. The first customer of this new concept is MasterCard and it exists on both the web and the mobile applications (Android and iOS) of Twitter. Eventually, all the users have MasterCard as a promoted entry in their respective Following list.
Because MasterCard is promoted in your Following list it does not mean that you are following it. The account has only 388,000 followers and they have managed to get the ad inserted in every following list luring you to follow them, though they do not follow you.
Of course, having MasterCard in the Following list is not an issue, but the problem is you will get to see all the tweets made by the account. This is a concept of advertising that they have chosen and you will find MasterCard everywhere on Twitter following lists.
It is suggested by some that blocking the offending account will remove MasterCard from the slot in your following list, but it will be replaced by another such entry. Though you are not following MasterCard or similar accounts, it is there in your top ten most recent follows.