Fast.com frequently tells me speeds that cannot exist. Why yes, I am definitely getting 1.2gbps over this gigabit Ethernet and 940mbps Internet connection.
It's hard to guesstimate the exact bandwidth of the data that arrives in your browser because of differences in protocol, MTU, header compression and all that nonsense, especially over technologies like WiFi. In my experience, the WiFi throughput numbers seem spot on.
Their compensation makes for some hilarious statistics, but when you're downloading more than 200mbps the fast.com speedtest doesn't make much sense anyway. No way in hell is Netflix going to allow your single home internet connection to somehow pull in a full gigabit of streaming video. If you're your own ISP you can make it happen, but on the other hand you'll probably also know how to get statistics directly from your network hardware, in which case the fast.com numbers are useless but it still becomes a useful way to spike the load.
Netflix's speedtest is mostly reliable for what it's meant to do, which is solve the question of "my internet is fast but Netflix keeps buffering".
Edit: another factor to consider is that accurate timing has been disabled in most browsers because of side channel attacks like SPECTRE. It's possible that those are affecting your measurements at very high speeds.
And of course, I'd not expect them to want me to pull down 1gbps of streaming video. But I would expect them to burst my connection when downloading videos for offline viewing, their own bandwidth permitting. Unused bandwidth at a point in time is wasted bandwidth. But more central to my original point, I'd also expect their browser-based speed test to not claim I'm going faster than is physically possible. Let's be honest - however they're measuring the actual connection speeds is not as accurate as some of the alternatives on the same playing field.