Plenty of variables can explain for that discovery hypothesis....

Plenty of variables can explain for that discovery hypothesis. For one thing the technology for exploration is slowly but surely improving - the first/second tenement holder from 1960's statistically have no chance for a decent discovery against modern techniques as well as nation wide mag/gravity databases we have out there for the general public as reference. Also some commodities weren't as relevant as a few decades ago which meant we weren't looking for the same things - lithium comes to mind as an example for that.