Thanks for reaching out. At the moment you subscribe, as a pro user, we try to get as much Pro Stats for your account from Twitter. Unfortunately Twitter only gives very recent data to work with (most of the times a couple of days). However, we will store all the information we're able to get, so in a couple of months we have Pro Stats data from the start of the subscription.
Do let us know if you have any further questions. Thank you for contacting Twitter Counter!