You are like a snowflake. Your muscles and bones make up a unique body that does not identically match anyone else. Therefor your body mechanics will be different from every one else.
Your skills, competency and capabilities will also be different from others. Furthermore your aptitude, or how much you like doing something, will differ. All of these aspects affect how you do something.
To maximise the efficiency and comfort of the task, tools and procedures need to be custom fit to the user. But there is often a trade off in cost and availability. Let's look at a real world scenario, the bowling alley.
When you go bowling your job is to throw a ball down a lane to knock down pins. There will be a wall of bowling balls for you to use. There's even a good selection to help tailor the ball to your body. There will be different weights and finger hole sizes to accommodate most everybody.
That ball will do the job for you. And depending on how good you are at bowling it might do incredibly well. Even better, those balls are cheap and easy to get. But if you really want to bowl you need your own ball.
A custom fit ball, much like custom fit golf clubs, will improve your score right out of the gate. When a bowling ball is fit to your hand, and the body mechanics of your throw, your score and your accuracy will improve. You may also find that you are capable of handling a heavier ball with complete control. The trade off is that it is harder to get (you have to have a pro measure you and drill a ball) and costs more.
Most cases of ergonomic tools have more iterations than just the two extremes of generic and custom. Most items in the market place are actually designed for some group of users that can use it with no significant difference in task accomplishment. And most people can find something that works reasonably well for them. But to get the most out of something it needs to be an extension of you and that's as personal as it gets.
