There are many factors involving speedtest sites. Some servers aren't equipped for high speed tests. If you test 10 different servers you'll get 10 different results.
Then the subscriber side there is other potential factors. Like antivirus software, computer processors and RAM, and the browser being used.
The whole point of gigabit is for bandwidth sharing for multiple users. Not for any one device to get a gig. For example, you only need 25 mbps to stream 4k video. If 5 users are streaming 4k is only 125 mbps. Gigabit is way overkill and not needed for any specific device.