That is so true! I worked with an old timer when I started and I was as green as grass. Took me a couple of years to get going. After that he and I working together could get 3 man days worth of work done in 1 day.I will tell you this. My brother used to manage a tire store that also did a lot of other work such as tune-ups, batteries, exhaust work, radiators etc. Pretty much everything except engine rebuild. All of his mechanics got paid out of the book. Some of his mechanics would work 40 hours in a week and get paid for 30 because that's how much the book said the work they were doing was worth. Others worked 40 hours and got paid for 60 or 70. This happened on a consistent basis.
The point is the guys who are really good can do a lot more than the guys who are under motivated or under experienced or just not very good.
The standard hours for anything are a starting point based on a mixture of the good bad and the ugly as far as the people doing the work. Your guys may be better or worse.
Incidentally, my brother also told me that the guys who tended to get more done usually effectively worked in teams. He was convinced that two guys could often do the work in less hours than one guy because the guys that helped each other and work together were the guys that were really productive.