Thanks for reaching out. Here at Twitter Counter we track over 67 million Twitter accounts, as you can imagine this requires a lot of resources. To keep everything to a minimum we only track a 'normal' account every now and then, if we miss a couple of days of data we try to calculate it as accurate as possible.
Premium accounts will get tracked every hour, so they are a lot more accurate. Besides that we're use caching, this means that you may see data that is up to 24 hours old.
Now that you've subscribed to a Premium account we will store all the information we're able to get, so next month you'll have accurate and comprehensive data from the start of your subscription.
If you require more information about your premium subscription, please feel free to email us at firstname.lastname@example.org.