There are many reasons why estimates may seem far apart.  Both repair facilities and insurance companies may provide written estimates.  Normally, the difference is caused because all of the same items and operations are not estimated.

Estimates may in fact be very similar.  One estimator may feel that a hidden item is damaged based on his past experience and he/she may include the repair cost in the estimate.  Another estimator may simply estimate only repairs that are visible before the vehicle is dismantled.  What is most important is ensuring that your vehicle is properly repaired and safe.  Also, in most cases the final repair bills will be very similar.