Reading music is like reading a second language. When you are reading a second language, your brain equates the words you are reading to the words you know from your mother tongue, whether consciously or not. When you are reading music, your brain must equate the symbol on the page to a note or finger position in the same way.
If reading music is like reading a second language, then transcribing music is like translating from one language to another. It is not often the case that the music available for an orchestra is written in the correct key for every instrument. I play a B flat clarinet and sometimes find myself reading music written in A or C. This means that the notes I see on the page are not the notes I must play! To keep it as simple as possible, I imagine I am playing in a different language to that in which I normally play and ‘translate’ the notes as I go.
To go one step further, moving from one second language to another after a long day is like switching gears in your brain. Similarly, moving between transcribing music from A and transcribing music from C from one piece to the next is like switching from French to German – neither of them are my native tongue and I don’t always know all the words, meaning I can get tongue-tied!