Historical reports include all posts shared publicly on Twitter and/or YouTube during the specified date range. (Facebook and Instagram do not provide historical data.)
Historical reports don't include posts shared privately, or posts where the hashtag isn't in the original post (i.e. if it's in comments, if it's added to the post later, etc.).
Your report exist as two things - the data is presented in an online dashboard similar to our real-time trackers, as well as a detailed Excel report similar to this sample file.
Your Historical Data Report is a stand alone report, and will not track real-time posts moving forward.