This is because the activity is not yet complete. It puts the proper date when the activity is completed.
Until then, it uses the placeholder of 0 which refers to the start of the Unix Epoch time https://en.wikipedia.org/wiki/Unix_time
which is UTC January 1, 1970, 00:00 hours. Moodle stores dates internally as the number of seconds since this time.
(You see 10:00 AM on your report due to your timezone offset from UTC/GMT; some users or sties may see dates of December 31, 1969 due to the offset.)
If the gradebook date is accurate, that indicates the user got a grade at that time, but that the activity is still not completed. There may be other criteria for completion such as the grade may not be a passing grade, etc. depending on how you have set up the activity completion for that activity.