The last app that is showing cannot be hidden. When you quit the last app, such as Safari, and re-open it, then it is no longer the last app showing, you will find that the Finder is also showing (and can be hidden) and you can hide Safari. If you open another app, then hide that one, then you can't hide Safari, as it's the last app showing.
Bringing Finder to the front will let you hide Safari. This is completely normal behaviour.
As I read this, it doesn't make much sense to me, but that's how 'hide' works. You have to remember that the Finder is also an app that can be hidden.