Last backup activity
Status message that indicates the last date backup activity of any kind was detected on your device.
Last completed backup
Status message that indicates the last date a backup reached 100% completion to the specified destination. If there are multiple backup sets backing up to the same destination, the date shown corresponds to the most recent backup set that completed.
If there's a large discrepancy between these statuses, see Why is my backup not completing? for troubleshooting steps.