ANP no AVA: Can someone explain why labor costs vary so much between garages?

I recently got two quotes for the same repair on my sedan. One from a small shop near my apartment and one from a place in Mussafah. The price difference was huge, mostly in the labor charges. I am trying to get better at understanding workshop pricing in Abu Dhabi so I don't overpay next time. I read that some shops charge a fixed hourly rate while others estimate based on the job, and that diagnostic fees can sometimes be hidden. There is also the factor of whether they use original parts or aftermarket ones. It is a lot to take in. For those of you who have been here a while, what is the biggest thing you learned about how garages structure their prices? Are there certain items on a bill that you always question?