Something puzzling

In our own language, we don't process every word that we hear. Neither do we process every word that we hear. In fact, I have on good authority that we only actually process 1 in 6. So there's a lot of predicting and guessing going on. This approach must be more efficient overall.

And yet we are advised, when learning another language, to choose books that we almost completely understand. Paul Nation, is believe, is responsible for the advice that 95% of the words on the page ought to be comprehended. That's just one word out of 20 which is new.

Doesn't this strike you as somewhat anal? What are we so uptight when we look over a text in a foreign language? Where does that meme come from, the one that tells us that we need to be practically perfect and look up every word?