How much the pasta cooks after you drain it varies a lot depending on what you do to it after. For packaged dried pasta, I find the time on the box is only right if I rinse the pasta after draining it (or am doing a dish that involves adding a lot of room-temperature sauce); I'm too lazy to do that, so I subtract a minute if I'm using an already warm sauce.
Where did this idea come from?
I used to believe this too and did exactly as you suggest. But I always left it in longer.
So I started following the timing on the pack. It always works for me for normal dried pasta.