The episodes have different duration. Consequently, the duration of each episode is listed at the episode level. In that case, the duration is set at the episode level.

The episode duration at the series level is deprecated and should only be used when all the episodes have the same duration.

That doesn't seem to be how it's done with most other shows on this site, then it appears to be the average length.

Feel free to report the shows where this field contains an average value, so we can remove it.