I think it does take longer to go from adept to master as it does from trainee to adept - the villager doesn't always gain skill each time they are successful, so they need to work a little harder to keep increasing the bar.

(I don't know how it works officially, but it reminds me of some RPG games - in those games, your chance at being successful with a skill is a percentage. Your chance of gaining skill, if you are successful, is equal to 100-your skill. When you have a low skill, there is a good chance you will gain more. When you have a higher skill, there's a lower chance you will gain.)