Glad this is useful to someone.
In my experience, the pixels/inch number is useless until you want to print*. You have a set number of pixels; if you scale the image, you scale the total number of pixels.
You can change the pixels/inch but it makes no difference to digital image quality - it's still the same number of pixels after all. But, when you print, or use print preview, the printer will use the pixels/inch number to size the image on paper.
BTW, the Full Size image I uploaded, according to my editor, has 72px/in so I'm not sure why yours reads different.
* Some display programs might take a cue from the pixels/inch number to determine default display size, but the programs I use default to fitting the image on screen, with 100% "size" being 1:1 pixel mapping with screen resolution rather than pixels/inch.