What determines "Hide physical URLs from search" and why does Search have a problem with it?

Publishing Pages have a property called Hide physical URLs from search. In the many different SharePoint Online environments I help manage can see that some pages have this set to true, some false. I don't know why this is, but I'm pretty sure no one has been changing these manually. This property is also a problem, since SP Search often does not pick up pages if this is set to true.

 

Questions:

1. What determines "Hide physical URLs from search"? Why is it seemingly random or changing?
2. Why does SP Search sometimes not pick up some pages where this is true?