If that router has a 1Gbit port it’s physically impossible and likely a measurement artifact.
Actual speed on a 1Gbit port is something like 940Mbps according to experience (I believe the theoretical max there is 970).
Wired Ethernet is typically full duplex.
Not sure what GP’s situation is, but I have a 100Mb/s fibre internet package but all hooked up to 1Gbps capable equipment on my side.
My typical speed test results are around 104Mb/s. Before being upgraded, on the 50Mb/s package I was getting 52Mb/s.
My suspicion is that fibre network operator (OpenServe in South Africa) applies rate limits which are technically a little above what their customers are paying for, perhaps to avoid complaints from people who don’t understand overheads.