Looks like Rhodysail has it exactly right.

What I see a lot of is someone tacks in front of someone near A mark. The person behind has time to avoid as long as they start avoiding as soon as the boat starts it's tack. The problem being that they shouldn't have to start to avoid the boat until the tacking boat has completed it's tack.

I don't quite understand why the rules people put in the phrase "above close hauled". I think that gets sticky a lot of times. Most boats don't have to go above close-hauled to avoid a tacker in the zone. Most boats at the end close to the mark start reaching some to get there. That leaves the door open for boats to tack in the zone.