Knowing how to do a push-up and doing thirty are different skills. Same with everything you “know.”

Knowing the shape of a thing and being able to sustain it are different achievements. Understanding the movement takes a minute; the capacity to keep producing it under load takes months of unremarkable work. This applies far beyond anything physical, since most of what people say they know is the description rather than the capacity.

Test the gap somewhere it is easy to measure. Pick a thing you say you know and see how much of it you can actually produce in one sitting. The number is usually humbling and always useful.