True. This all is because of lack of technical education. The mechanic is not actually completely wrong though.
What they do is they compensate for other discrepancies in engine. For example, if your valves are recessed in the seat, having the same clearance will result in less valve lift. This may be observed under highway conditions. So what they do is they tweek the parameters so that vehicle keeps on going in the city drive.
And we all know (or have a phobia) that opening the head would result in expenses and we need quick fixes at times. This mentality is what they cash.
This all could be avoided with only the know how of how to use, compression gauge, vacuum gauge (it can do wonders if properly read) and an oil pressure gauge. Plus the excessive use of a torque wrench. The whole concern in public that "an opened head can never be factory like" will only then be rectified.