Thanks for reaching out. Here at Twitter Counter we track over 21 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 once a day (or sometimes a little less), 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.
If you wish to have correct numbers I would advise you to hit the "Update stats now" link at the top of the page every once in a while.