It is a highly flawed system though. Body shops are encouraged to underestimate the repair to get the job. They then get the job and start RACKING on the additions. We had one car that was estimated and repaired. The final bill was nearly double the estimate and was actually more than the vehicle was worth. It should have been totaled, but that would not have put money in the shops bank account.
Outside of the auto body shop world, most contractors are held to come in very close to their quoted estimate.