Ok having been a mechanic for over 30 years I have come across this many times and now explain to customers about Book time, or did until I started working for the council on their fleet vehicles, no customers to worry about now. So this is a totally unbiased view. The book time is the time the manufacturers state it will take to do a certain job, but that does not include any problems that may occur along the way, i.e seized/ broken bolts worn bolt/nut heads. Or the worst of all owners modifications by which I mean they have had a go themselves and realised they can't do it and put it back together with cable ties and duct tape, or half the bit of the engine is in the boot. Don't laugh it happens.
You must also realise that the main dealers are mostly working on the same makes and models every day so get to know the jobs and more importantly the short cuts. This is why they can give you a fixed labour price for a job as it's more than likely they will get it done in the time stated. But, you are charge a premium labour rate for that privilege which covers the dealer backside should they come up against any problems.
Please cut your mechanic a little slack as he probably didn't charge you for all the time he was working on your vehicle anyway. Plus just imagine is you had to pay a mechanic the same rates as say a plumber or electrician, which in my mind they should be charging. As if a plumber mucks up he/she may float your house. If a mechanic mucks up he/she may cause an accident, or worse may kill someone.