Andrew,
Your DEP:ConRl is definitely reducing the ATP quantity. If that object did not exist, your ATP qty would be much higher.
A contributing factor to your 'high' ATP quantity are those old Purchase Orders.
You probably should consider doing something about those Purchase orders which were expected to be received in 2011, 2012, and 2013. I don't know much about your vendors, but for vendors I have dealt with, if 5 years goes by and they haven't sent the goods yet, they probably aren't ever going to send the goods.
Best Regards,
DB49