Why does the Apps window say "summing caches" at the bottom?

Short answer: It's still adding up your apps' caches for the combined total. It works in the background and shows the figure when done.

Storage Studio is still adding up the caches that belong to your apps, so the combined total at the bottom of the grid isn't ready yet. It works quietly in the background and fills in the figure when it's done.

The On this Mac and Fast drive app totals beside it are already shown.