Michael Gilmore of Park Logic has studied domain name monetization more thoroughly than about anyone in the industry. He has been able to see inside different parking companies and two recent articles on his blog provide a great perspective on the domain industry over time. Check out his post "Critical Insights Into the Domain Industry" and the follow-up post.
instantly grant a number of additional domain feeds to new parking companies. Some of these feeds had a honeymoon clause that allowed them to have a competitive advantage versus the larger incumbents. Many domain investors flocked to these new companies as they were seen as their salvation to paying renewal fees.
I always wondered how the "new kids on the block" were able to boast better parking revenue. Gilmore's perspective is without peer and his ParkLogic service comes highly recommended for those with large portfolios.