I couldn't get the report to load so had to borrow the magazine. I would not worry to much about the depths quoted, more important is that it was pulling out hammered silver that all the other machines had missed. The coins are about the size of a one cent U.S. coin but much thinner. Many detectors would indicate them as foil.
In general the U.K. has less mineralisation than the U.S. but three thousand odd years of iron losses from fragments to chunks of broken plough add their own problems.