I'm a programming language designer, so I think about this stuff all day every day.
I believe you're right that expertise has something to do with it, but I don't think your comment quite connects why consistency leads to expertise.
Consistency is about taking what users already know about a system and compounding that expertise across other parts of the system. When a system is consistent, once I spend the effort to learn X and Y, then I also know Z and W because they'll behave the same way. In an inconsistent system, I can't extract as much value from my existing learning investment.
I fully agree. it's also how the user takes what they know and making it work for them.
Windows 95 had years of user testing behind it. They'd sit people down who were used to DOS or Windows 3.11 and give them tasks and see where they struggled or where it worked and iterated on it and the talks by the ux teams really stuck with me. every color and design choice was important even the awful but good real background wallpaper colors
I believe you're right that expertise has something to do with it, but I don't think your comment quite connects why consistency leads to expertise.
Consistency is about taking what users already know about a system and compounding that expertise across other parts of the system. When a system is consistent, once I spend the effort to learn X and Y, then I also know Z and W because they'll behave the same way. In an inconsistent system, I can't extract as much value from my existing learning investment.