My initial response was "What am I missing here?"

Then I found it: "It should be noted that, in addition to the re-distribution of ounces from previous pit-constrained models to the new underground constrained resource, the parameters chosen for this updated pit-constrained resource are more conservative than parameters used in previous updates. This results in a more robust overall mineral resource with a focus on higher-grade material at the expense of lower-grade open pit material. Consequently, the pit-constrained resource now contains fewer ounces."