There are many things to consider;
A service is also (as RobW rightly mentions) an inspection which a knowledgeable technician knows exactly where (and when) to look/check a particular component.
However, many dealer technicians are not happy with dealer standards and set up on their own or join independent garages where they are not restricted by time limitations and can there for carry out a more thorough job and do those little extra jobs that the customer appreciates so much.
You service book could be full of dealer stamps but the service operations were poorly or not carried out. An independent stamp does not mean it has recieved poor servicing....but how do you tell, you can't!