On PVC Diamonds are rarer to find than vanilla. This caused a situation where all the existing caves have already been explored, and the underground of older parts of the map is literally drilled by thousands of 1×2 level-11 tunnels. If Diamonds are rarer, you need to mine longer tunnels, and long tunnels usually cross already explored (or Claimed) areas.
We therefore tweaked the ore generation system to make so that Diamonds will be able to generate at any height.
This change greatly benefits the old Quarry mining system.