default and custom gridline settings

Could somebody explain in simple terms how to set custom gridlines?

From "Help - To set the spacing of the gridlines" -

"If you want to change the grid spacing intervals or the number of lines displayed per unit of measure, choose an option from the list box. The options are based on the unit of measure used for the ruler."

List box? What list box? The options are based on the unit of measure used for the ruler? Really?

OK, I open an image. The default is 200 x 200px. I change it to 75 x 75px. Save. Open image. Default has reverted to 200 x 200px.

I open another image. The default is 72 x 72px. I change it to 100 x 100px. Save. Open image. Default has reverted to 72 x 72px.

I open another image. The default is 96 x 96px. I change it to 200 x 200px. Save. Open image. Default has reverted to 96 x 96px.

I bring in all three images. The "default" changes respective to whatever image I select. The settings for the rulers are the same in each case.

So, 1) is there such a thing as a "universal" default/custom grid setting for all images? 2) is it possible to set a custom grid per image? 3) why does the grid default to a different setting per image?