Depending on the quality of the lock there is some play in the mechanism (that is also why lockpicking works). This reduces the number of combinations. If you can lookup (model type documentation) or learn (by feel) how the internal mechanism works you can reduce the amount of combinations to guess even more as you can approach the combination one 'layer' at a time.

I once witnessed someone open a safe in a few hours. It is truly amazing to see these kind of proffesionals at work. How they approach this puzzle and the knowledge they can acquire by just feeling. Especially considering it took me a few tries to open the safe when I knew the combination.