Ok, but if you put 15 amps of solar on your hypothetical circuit, which is 3-5 panels, you would not trip the 20A breaker.

Then someone puts two space heaters at the end of their branch circuit, they could easily exceed 20A without tripping the breaker and without straining incredulity.

And you are right, 40A was a silly choice, I figured bigger numbers made it more obvious the problem. But it's not hard to construct a likely example on a 20A circuit either.