Zenyatta Ventures Ltd

The reason is that a geological resource is the theoretical maximum amount of mineralized rock, whereas there are economic factors that make certain mineralized zones uneconomic, even though they are above the cut-off grade.

The criterion involved is "the reasonable prospect for economic extraction", which must apply at every stage of the assessment of the project.

This video, from Eric Sprott's team, gives a complete examination of how resources shrink under economic considerations.

http://sprottglobal.com/natural-resource-investing/investment-university/ore-deposits-101-part-11/

Lar