Explanation
Six seconds on a nine-second video and six seconds on a sixty-second video are opposite results, which is why watch time in seconds tells you almost nothing on its own. Dividing by the video's length gives retention as a percentage, and that is the version that compares across posts.
Watch time exceeding the length of the video means people rewatched it. That is the strongest signal short-form produces, and it is the reason a video designed to loop can accumulate more total time than a longer one that nobody watched twice.
The common mistake
Comparing watch time across videos of different lengths. It is the most common analytics mistake in short-form, and it consistently makes short videos look worse than they performed.