I am not sure where this feature is, but I assume later graphics cards have a state or mode to perhaps accelerate RAM with a trade off: FGenericPlatformMemory::SupportsFastVRAMMemory | Unreal Engine Documentation
From the link:
Gets whether this platform supports Fast VRAM memory Ie, whether TexCreate_FastVRAM flags actually mean something or not
The Fast VRAM appears here as well: ETextureCreateFlags | Unreal Engine Documentation
TexCreate_FastVRAMPartialAlloc
The texture can be partially allocated in fastvram.
TexCreate_FastVRAM
Texture needs to go in fast VRAM if available (HINT only)
It could be that normally the information is stored on RAM, but having it on the VRAM makes it run faster.