Some stutterers stutter so lightly that it is only heard in certain situations, or use fluency techniques to better hide their stuttering. Others have a covert stutter, and change/delete every word in their sentence that they might stutter on, which can be very mentally tiring too. These people may also have a misunderstanding of stuttering, and may think that typical disfluencies are stuttering. In short, lots of possible reasons, try to talk about it with them to remove doubts otherwise.