It's the drivers but it was a relatively recent addition, think it was added when either the 30xx or 40xx series shipped and the lower cards had pitiful VRAM so they enabled it by default so they'd work with all games.

Most people who know it does this turns it off because it kicks in too early so if you have 24GB it'll offload to RAM and tank your inference speed when you hit around 22GB use.

https://nvidia.custhelp.com/app/answers/detail/a_id/5490/~/s...

NVIDIA's GPU drivers on windows 100% do this

https://i.imgur.com/c0a3vUy.png

The Nvidia driver has used system memory fallback for a couple of years now.

https://nvidia.custhelp.com/app/answers/detail/a_id/5490/~/s...