Marked To-Do as completed but it still sent notification, why?

Yesterday, I created a To-Do item to remind me to check on something today. Remarkably, I remembered to do it on my own this morning and then went into my To-Dos and marked the item completed (at 8:47AM) At noon, I was reminded that I had a To-Do item. Why was I notified if I had already marked it completed?

Thanks.

Created on 3/3, set as due on 3/5, notification should be displayed 1 day before (3/4=today) with sound and e-mail