Why is the estimated PAI for my run so low? I’ve done a similar run in the past with a much higher estimated PAI?

Remember, your PAI is an indication of the health benefit your recent activity has given you. If you haven’t been very active, a workout will give you more PAI because it has a real impact on your health. If you’ve been extremely active recently, the same workout will have less impact on your health (although it will still have all the benefits), so you get fewer PAI for it. If you get a low estimated PAI for a workout, that’s not a bad thing. It just means you’ve been very active recently, and your score is likely to be quite high anyway.