This is very obvious any time you try to create exhaustive lists of something.

For example, for every country in the world, I would recognize it and say, yeah, thats a country.

But if I had to write all ~200 countries into a list, I would probably miss quite a few.

Or, if you gave me names of all US presidents, I would for each of them go, oh yeah, thats a president. But ask me "name all the presidents", I wouldnt get further than 10.