If you you've deleted the app, then all it's stored data is gone. It may take a bit for the About data to catch up with the fact. You can try a restart to hurry the process along, or a sync (if you do regular iTunes syncs) will usually free up the space.
Restart: Hold down the power button until the power off slider appears. Slide it off. When the iPad finishes shutting down, hold down the power button again until the Apple logo appears.