Potential activity is dependent upon on how many activity period that account has posted at least single post. Each activity period is 14 days long. If you post 14 posts in those period your real activity will increase with 14. If you post only 1 post then that will be added to potential activity which can be gained by posting later
Counting potential activity is a very lazy work
There are many websites which can tell the exact amount of potential activity account contains
http://www.bctalkaccountpricer.info is a free website which provides those info with other basic info too
While
http://www.bitcoinrates.in/bitcointalk/getactivity.php is an advanced version but not free
Hope this helps