Here’s another way to look at it
Imagine someone decides they want to learn a new skill, for example project management. They pick up a book, read several chapters and use the limited information for a new product launch. It works well in their opinion.
The person feels they have got to grips with the skills required for project management fast. When asked how competent they are, the person might say 75%.
At this stage of the learning process, the person underestimates the size of the overall knowledge base within project management and how much more there is to learn.