Wikipedia article [1] states that differential GPS can provide up to 10cm accuracy.

Differential GPS works by having a local GPS receiver station which location is well known. This station is comparing the location determined from the GPS signal to its known location, calculates necessary corrections and sends the information to other GPS receivers.

[1] http://en.wikipedia.org/wiki/Differential_GPS

You are incorrect. It's quite possible to achieve sub 10 cm level accuracy with an RTK based GPS system [1]. The problem is that you have to install base stations to gather the corrections and then have the radio towers to distribute them. Since mining sites are fixed locations, it's pretty straight forward to setup up the needed infrastructure for an RTK system.

[1] - http://en.wikipedia.org/wiki/Real_Time_Kinematic

I think they use local calibration. You have a GPS receiver at a known, surveyed point onsite. You compare the GPS placement to the actual location of the receiver, then use that difference for calculations being performed on other receivers.

The military could be achieving that accuracy, though.