Or hydro can't put that much over the spillway because of regulations (environment, down stream flooding, etc...) - it has to go through the powerhouse.

Or (for hydro again), there was a release of water upstream in anticipation of demand (it takes a few hours to go from the Grand Coulee to the bottom dam) and that demand didn't appear (or other sources produced more - such as a sunny and windy day).

Hawaii and its island power issues also has the "well, you can turn off the centralized solar power systems, but the distributed residential ones are difficult to curtail."