Xray, yes, it's possible that persons passing shallow clad *could* pass a shallow goodie. But it's all in the odds: if 99% of the time, a shallow zinc penny turns out to be ....... a shallow zinc penny, then that speaks for itself.
As far as masking, it is true that if you strip-mine out all the surface targets, you will eventually find things that the shallow stuff masked. Or put another way: some persons bristle at passing ANYTHING conductive in the turf (no matter how blighted or junky the turf is) because in their mind, you can have both the old, AND the new, if you "dig all". And they rationalize that sometimes an oldie is shallow (brought up by a gopher, etc...) and therefore, you can have "the best of both worlds". But invariably, here's what happens when you turf hunt in an old park, with someone using that mindset: At the end of the hunt, the person honing in on only deep coin-type sounds, will end up with 10 or 15 oldies (wheaties, silver, etc..) and only 3 or 4 clad. The person digging all will end up with 50 clad, and 2 or 3 oldies.
Now I dunno about you, but I'd rather have one silver dime, than 100 new ones, etc... The reason why the person digging all the shallow stuff will end the day with less oldies, is two-fold: 1) his ears become subconsciouly tuned to only the loud "bongs", and he ends up being less receptive, and thus subconsciously passing, the deep whispers. Contrast to the person passing shallow clad, he ends up ultimately becoming more receptive to just the deepies. 2) the clad hunter ends up spending lots of time on the ground digging and probing and such, and lessens his time that he could have spent, honing in on only oldies.
As for the "sometimes oldies are shallow in the turf", it is the exception, not the rule. So just as in black-jack, you "hold" when you have 20 in your hand, EVEN THOUGH the next card *might* be a one. You're following odds. And naturally this is all for stratified turf, where age is usually commensurate with depth.
Now turf jewelry hunting would be another matter altogether, as .... of course .... jewelry can be modern. There are still blighted inner-city urban parks where you would go psycho trying to dig all the foil and beaver tails out, in search of the elusive gold jewelry. I can imagine odds of worse than 100 to 1 in some parks I have been to. So in some turf, I'm strictly angling for old coins, and save my jewelry hunting for beaches, where jewelry odds/ratios are just better, to begin with.