I think I've found the answer, still investigating the details. This write-up comes from Excel Tip of the Week #53 - Causes of formulas not calculating:

Cause: The workbook has been placed in “Manual calculation” mode. This will mean that Excel doesn’t automatically update all formulas whenever the workbook is amended and needs to be manually forced to do so. This also means that if you copy and paste a formula (like the addition formula shown on the right), the result is copied instead of the correct answer until the manual recalculation prompt is used.

Fix: You can either force a manual recalculation, or turn the calculation back to automatic. Note that manual calculation is usually set when the workbook is very large and unwieldy to speed up calculation times.
I colored the relevant text...

Russ