Actually, Bianchi, not entirely accurate on that one. It APPEARS "beyond level" as you say, because the top tube is slanted upward, which means it's not in line with the top tube. But it's actually level. If his garage door is level, I enlarged the photo and it's exactly level. So on that particular bike, -17 makes the stem level.
HOWEVER, I'm being OCD (which is the point of this thread, right?) because I also don't like that much of a stem angle either. -6 or -10, as you say, would look better. Oh, wait, THAT'S the point of this thread...