Make sure that the same version of libiwinfo and libiwinfo-lua is installed. The figures above look wrong and happen when the ABI between both libraries does not match.
The rate shown is what has been negotiated for each device based on device capabilities and signal quality. This is the bit rate that would occur if the transmitter in the router were transmitting to that one device 100% of the time. That condition will never actually be reached. The transmitter has to wait for response packets from the device, other devices, signals from other networks, and of course it can't transmit until the data arrives from the Internet service.
The router doesn't know how fast the ISP is. You can look at actual data throughput on the Status-Realtime Graphs-Traffic page.
The data shown by LuCI is derived from the iwinfo wlan0 assoclist CLI command. That command polls the kernel driver. Not all kernel drivers report full information.