Is this anecdotal or is it well known?
But most of the time, it's due to how these guys are managed by their client. When you bring in an individual from a top consulting firm, they will fight tooth and nail to get the results you want. Or at least not "fail" at getting those.
If this means doing a great job, they will definitely do that. If - due to factors they can't really control - they need to endanger the rest of the project so that their part is successful, they will avoid to do it if they can. But they will do it if they need to. Failing is just not an option.
This is the danger of hiring consultants: incentives are not fully aligned between you and them. You want the whole project to be successful - on the long run. They want their part to be successful - right now.
If you are careful enough, these two absolutely coexist. If you aren't it can be dangerous for you whole organisation.