It is a shape issue - thin and elongated. That's why the double blip - elongated ferrous items act like bar magnets with a field effect/signal at each end. And the double blip trick does not work at depth, or if the item is oriented anything less than horizontal. With bottle caps its being round plus alloys. Any anything round with a hole, like a washer - good luck. These ferrous demos ignore a simple fact. Use a PI around a lot of ferrous and you are going to dig plenty, and at good depths