True story: Back in the late 1970s/early '80s, before TID of any sort (disc. was just an ascending/descending dial), a friend of mine was getting a lot of silver in the parks with a 6000d (revolutionary at the time
). He would just crank the disc, so he wouldn't have to dig foil or tabs, and just go for the silver. After pulling a few hundred old coins from a certain oldtown park, he reasoned that there "must be some old nickels and gold jewelry I'm missing". So he sectioned off a particular area that had been good to him on silver coins, and spent several hunts turning down the disc. and digging all the low hits. At the end of several such hunts, all he had to show for his efforts was a few worthless orange buffalo nickels, and perhaps a single gold something-or-another. He reasoned it simply wasn't worth the embarassment of strip-mining in turf, when he could just go to a swim beach (fresh or saltwater), if jewelry were his goal. Naturally, at relicky places (oldtown urban demo's, ghost town type sites, etc...) he'd turn down the disc. But for junky turf, he reasoned that he was not going to be a hero.
I have seen the same thing today: I've hunted with persons in old junky parks, who bristle at the idea of passing shallow stuff (afterall, an old item is occasionnally shallow, or maybe it's shallow jewelry, etc...). They reason that "afterall, the clad adds up, and afterall, you can get both the oldies AND the new/shallow stuff, by digginng all". But invariably, by the end of the day, they'll have 1 or 2 old coins, and I'll have 8 or 10. The reason is that they spend all their time digging corroded zincs, surface tabs and foil globs, etc.... Their minds become subconsciously tuned to the "loud bongs", rather than the deep whispers of deep turf coins. It's all in the odds. Just like black-jack: when you have 20, you "hold", EVEN though the next card *might* be a one card, right? Why do that, if the next card *might* be a one? Because quite obviously, the odds are, it will not be a one card