The combination of the operating system (Linux/MacOS/Windows) and the particular hardware it has access to. With different hardware, /dev/(u)random has different qualities of entropy.
that's horrifying. I do slightly disagree with the final thoughts and works say that one of the xoshiro 256 generators is probably better than the 1024 ones. 256 bits of state is plenty to give you a longer cycle length than you will ever need (even considering birthday paradox) and the reduced state means it fits in a single cache line