Percival has rarely spoken more than a few words at a time. This is, perhaps, why Arthur is surprised by the seeming flood of speech that pours out of the man at Arthur's questioning.
"Of course Merlin has magic. You'd have to be blind and foolish not to notice how many coincidences happen around him. It's not a big leap from there. And he's a warlock, not a sorcerer. The terms aren't interchangeable."
Arthur gapes like a drowning fish, unable to form words. Percival just claps him good-naturedly on the shoulder as he leaves the training field for dinner.