Unclear who gotten the achievement, from API data

Calling the “Character Achievements Summary” API method.
This is done by specifying a realm and characterName.
I have a level 120 character and one that is not level 120 yet.
Both characters will state they gotten the achievement “Level 120”.
In the JSON response the “completed_timestamp” is filled-in for both characters.

In-game it shows like that as well. But! There a tooltip shows something like “Earned by: character 1; Not completed by: character 2”, where character 2 is the one logged in.

How do I determine if the achievement is completed by the characters I retrieve the summary for?