While all the firms in the market have access to similar resources and markets, some of them have been more successful than others. Even the predictions, organizational structure and staffing models are similar. As I see it, there are differences in how they develop three capabilities: process management, client-based capabilities, and coordination capabilities.
While most organizations get the appropriate CMM and PMI certifications, some have the complementary assets to successfully implement projects. Similarly, some firms have deep domain knowledge and client management skills that they make sure get utilized on each project. Finally, their ability to coordinate expertise, people and systems give them a definitive advantage.