I just spent two days at the beach, one of the major reasons I bought the M6, and made a point to apply what you have noticed. You are absolutely correct. If I bob the coil it sits firmly at hot rock or maintains a very low iron number and the tone id variances do not happen. This only works up until the 10" mark. After that the VDI stops registering when you bob the coil and tone id only responds part of the time.
It does however give you a VERY (90%) good indication that what you are looking at is indeed a bottle cap or something else with a high iron content. The beach that I hunted is generally very clean except for flakes of iron and lots of bottle caps down to about 10". The coins are deep as this beach has very soft fine sand that seems to go on for ever. I noticed one of the fluttering targets and tried this technique and found this time that it would still give me a mid tone with the occassional high ping. It was a very mdoern Canadian quarter at 10", the quarter has 4 pinpoints of red, rust crust on it.
It is a sucker bet to hunt this beach other than for the fact that it has very testing conditions for any metal detector. It is a great place to find out what your machine is made of. There are streaks of black sand and nails everywhere from decades of bonfires.
Since WWII and up until the early 70s this beach was very active with easily several thousand people on it on any given sunny day. There was a chip shack on the top of what was once a dune. The beach now sports several mansion sized cottages and no way for the public to get on it really other than an unpleasant walk from a parking lot that has capacity for maybe 50 cars. 20 or so years ago I went down there with a white's gold machine to hunt. My brother took over when I went for a swim. He handed me the machine back and said that he would be right back. He came back with a wheel barrow and a digging spade. In less than an hour he had dug up a small horsepower Evinrude outboard motor.
At the end of it all I am really impressed with how the M6 performed, it reached very deep for any metal and told me with a pretty good certainty what it was I was digging. I was digging all targets. my best performance dig was a flake of copper about half of the size of a dime and very thin from 11". I could raise the coil a few inches off of the sand before it lost the target.
Thanks for bringing this up, it was a great help.