I presume you run the speed test from a client computer that is connected to the br-lan? If so, then your result does make sense, even though the situation is somewhat strange. The SQM speeds are always from the perspective of the router.
For WAN-side, download is fetching from Internet, and upload is sending to Internet. For LAN-side, download is receiving from LAN, and upload is sending to LAN. And you've set the maximum speed to send into LAN to 5 Mb/s. All packets go through your router.
Thus, you are capping the maximum speed that your router is sending data to br-lan, even though it might receive it faster from the Internet.