You need to understand what an index of refraction is.
The listed refractive index of a medium is defined as the velocity of light through a vacuum divided by the speed of light through a medium. This order is important.
The speed of light through air is close enough to the speed in a vacuum that often people refer to the refractive index being the speed of light through air vs the medium, and it’s a close enough definition for most real time graphics. But understand there’s an inherent assumption here that light is travelling in air into a medium when you’re looking at those lists of refractive indices. When the light passes out of that medium back into air the equation flips. The index of refraction is now calculated as the speed of light through a medium divided by the speed in air!
This means using a single refractive index for something like a glass sphere in a real time rendered object is a funny thing. In reality, the refractive index for glass is somewhere around 1.5 for air to glass, but there are (at least) two refractions; the refraction entering air to glass (ior 1.5), and exiting glass to air (ior 0.67!). More complex objects like glass cups or heavily facetted objects make this even more interesting. For real time rendering you’re usually only simulating a single refraction, so an ior of less than 1.0 may give a closer approximation to the expected visuals than an ior over 1.0.
It also kind of doesn’t matter, since Unreal is just offsetting the screen position by some hand-wavy amount based on the normal and ior, just do what looks good.