Jeff,

This is a good question and one that is understandably puzzling. As odd as it sounds, 78 musket balls will indeed fit in 12-pounder case shot. These are the stated amounts in period ordnance manuals. If you look at some radiographs as on Melton's site, you can try counting the lead balls--it looks about right.

Standard 12-pounder canister balls were much larger in diameter than the case balls.