A SUM that runs without an error can still be wrong. When a total doesn't match what you expect, check these in order:
- Numbers stored as text. They sit on the left of the cell instead of the right, and SUM skips them. Retype them, or convert them, and the total updates.
- A range that stops early or starts late. Click the total and look at the colored box: does it cover every row? New rows added under a total are a common cause.
- A subtotal inside the range. If the range runs over a subtotal row, those numbers are counted twice.
- The total inside its own range.
=SUM(B2:B10) in B10 refers to itself, a circular reference. Excel warns and shows 0.
Press F2 on the total to see its range highlighted on the sheet. That one habit finds most of these.