Voltage tells one story, and CCA tells another. Both are important.
Voltage can maintain high because the individual cells in the battery is not shorted by dendrites nor corroded. A low voltage means some of the plates were not connected, or shorted and not contributing to the output.
Cold crank ampere (CCA) shows how much electrons can be moved when needed. A badly worn battery cell can retain voltage but the amount of current generated reduced over time due to usage and recharge cycle. A large current will be drawn during engine start, if not enough current, starter won't crank.