Here’s the thing- your connection between one of your devices is composed of multiple links- you can think of them as pipes if you like. Each one has different capacity.
So your AT&T Fiber is basically the connection between your house and AT&T’s network. But if you’re using a wireless device, that may have different speed than your fiber connection. And within AT&T’s network, there may be hops that are faster or slower. And between AT&T and whatever site you’re going to may have hops at different speeds.
The kicker is that your connection is only as fast as the slowest one. Historically that’s almost always been the connection between your house and your ISP (AT&T).
But with a 1000 mbps connection there, you’re almost certainly going to start seeing bottlenecking somewhere else. Most wi-fi isn’t 1000 mbps. Hell, a lot of wired connections aren’t that fast, or aren’t that fast all the time.
For the sake of argument, let’s say you’re running your speed test from a PC that’s wired to your router with a gigabit ethernet connection (1000base-T) with a really short cable, so that you’re actually getting 1000 mbps through it. And let’s go further and say that your AT&T fiber connection is going at the full 1000 mbps as well.
If there’s a 500 mbps hop in AT&T’s network somewhere, even if the website is on a 1000+ mbps connection, and AT&T’s external gateway is 1000+ mbps, your connection is going to be 500 mbps.
squeegee is right too; IF your equipment is super-duper 802.11ac, and everything is just-so, you might get 1000+ mbps via wi-fi. But a lot of stuff isn’t 5 ghz yet, and a lot isn’t 802.11ac (or higher) yet. All of those are going to be slower than 1000 mbps and will be a bottleneck.