You're right, there's no way they would not tout the "15% grade" of the actual ore, if by some miracle there was 3.14% grade the entirety of their staked land.

You're right, there's no way they would not tout the "15% grade" of the actual ore, if by some miracle there was 3.14% grade the entirety of their staked land. Ore is by definition the "economically feasible/interesting" subsection of the rock covered by the claim.