When we're talking about the interfaces, we're obliged to take into account the devices that makes us possible to display the content we aim to see. So that the main medium when the term
responsivity was introduced was computer screens/displays and
As you might imagine displays of all the computers/notebooks, the screens are rectangular on default and people using these displays intend to make browsers/applications minimized via reducing it's width rather than the height as it opens more broader and wider area.
Doesn't responsive layout mean that my app should look the same in devices of all widths and heights?
Well, it's just because in these days mostly people are mobile users and the term need to be updated as the term
responsivity originally also covers the same issue. Or there might be attempts to describe the concept differentially as only mobile users ran into this seperation since web is mostly on rectangular screens.
If 2 devices have a lot of difference in HEIGHT, my app in one device
will look drastically different from the one in other device
Besides you're totally correct and even it's the same for width for development point of view, it might mostly happening beacuse the heights are used %100 to fit all the screen sizes for mobile preferably. But that's not my main arguement and point to avoid using responsivity as a height on mobile or web eventually.