Why Longer Skippable YouTube Ads in DV360 Have Higher Completion Rates

Hey everyone, I'm working with skippable YouTube ads on DV360, and I’ve noticed an intriguing pattern: our 15-second ad is showing a higher completion rate. Despite having various ad lengths in the same group, the longer ad is getting fewer impressions but surprisingly has a better completion rate. Does anyone have insights on why this might be the case and any tips for optimizing these ads more effectively? Thanks a lot!