You'd hope that people opting for gigabit+ internet services would have specific applications that they need that sort of connection for, at which point it's easy to verify if the service is working as expected (you just use one of those applications). If I bought a 1Gbps connection because it was only £5 a month more than 500Mbps then I'm not sure I'd be too worried if it 'only' ran at 850Mbps on random speedtest servers.
No, it's all about 'getting what you pay for' even if they're trying to run speedtests on old iPhones connected to a 2.4Ghz wifi network.
Honestly if an ISP made a site that just matched the IP address against the service/speed, it'd play a little graphic to simulate a speed test and show the 'correct' result most people wouldn't even notice. I've had people ask what's the point of having 1Gbps if they can't get it on a single WiFi device, I don't know, you paid for it!