Understanding how SS calculates completion

Sorry if this has been asked before, but I am not clear on how SS calculates % complete with the child-parent relationship. The example I have is that I have one child at 10% complete. Roll up task shows 0% complete. I changed the completion date from in the past for a missed deadline and the parent roll-up goes to 10%. I then edit the corresponding tasks that come afterward but where there are no predecessors set, so there is no interdependency in SmartSheet. I adjusted the end date for that and the parent changes to 5%. Why would the % complete calculation in the parent change? It's especially puzzling why sometimes I can have a parent show 0% with 6 or 8 subtasks at 100% and one subtask show 0%. This is especially frustrating when I might have upper management possibly only looking at the parent % complete for project status in my dashboard which I did explicitly for management review as I feel like my projects are much farther along than what the roll-up is calculating.

I have attached an animated GIF file to show what I mean. I have no idea if the animation will display from the forum, but I suspect that your browser must be allowed to run animated GIF. It's what I could do since the forum does not allow MP4 as a file type to upload.