wglGetPbufferDCARB with a NULL parameter returns a device context (hDC). But which hDC is this?

Question

Accidentally I put a NULL parameter in wglGetPbufferDCARB.

It was something like:

HDC hDC = wglGetPbufferDCARB(NULL);

Unexpectedly this code returned a real hDC! But I did not find anything about it.

Does anyone know where this hDC comes from?