[noselasd]

That says exactly why.

You read from a file and tell the OS to not buffer it(or you tell the OS to flush the caches before you start) on a native system, and it does exactly that.

You read from a file and tell the OS to not buffer it (or you tell the OS to flush the caches before you start) on a VM, and the OS thinks it does that, but the hypervisor living below the VM buffers some of the data anyhow, so you're really just reading from memory.