I haven't used an A51m myself, but have you tried checking the Intel HD Graphics Control Panel?  On many multi-GPU systems, the built-in display (and usually some or all of the display outputs) are wired to the Intel GPU, with the NVIDIA GPU using NVIDIA Optimus technology to act as a "render-only" device that passes completed video frames to the Intel GPU for output to the displays.  In that setup, the display's refresh rate would be controlled by the Intel GPU.

Thanks for the reply.

Yes, I did actually. The intel GPU also only has one refreshrate available, which is quite bizzare...