When you decode, you are paying attention to the elements you are reading or hearing and focused on converting them into your language, English, in my case.
This is not reading/listening comprehension. You do not decode a language you understand in order to know the meaning of the language. You grasp the meaning directly from the utterance or lines on the page…. or screen.
This is the damage done by the grammar/translation method. It is by no means restricted to Latin teaching; I have run into people taking Spanish who describe essentially a G/T approach to teaching the language. But it is the long-term effects of the G/T model that has damaged us.
The modern languages, in order to get a foothold in schools as academic subjects, had to follow the Latin/Greek model, which, at that time, was G/T. Through that has survived the notion that a language is learned by memorizing the rules that make up its structure and memorizing the words that make up its lexicon. By practicing these in one way or the other, the student is supposed to learn the language.
In the classical languages, there is still little notion that an ordinary mortal could read and comprehend those languages off the page; they are holy texts which must be carefully analyzed and, for full comprehension, translated into the student’s own language. Unfortunately, that still cripples the teaching of those languages and still exerts ill effects on the teaching of modern languages.