Oh! It's because the girl(Ran) had a boyfriend called Shinichi that went missing(doesn't show up a lot after an "event") At that same time, the small boy in the video(Conan) that showed up soon after the event. Because of some similarities, Ran starts to suspect Conan of being Shinichi. So she called Conan "Shinichi" when Conan was distracted, to see if Conan would answer and when Conan did, she was surprised as that would mean, Conan was Shinichi. (SO SORRY ABOUT THE 1 YEAR LATE REPLY!😅 I hope this reply helps! The show name is called "Detective Conan." In case you want to learn more about what had happened to turn Shinichi into Conan. Totally not trying to promote the anime :D)