Why is xfinity able to offer faster internet speeds than AT&T?

According to AT&T, the internet download speeds in my area max out at 25 Mbps, and that's the plan I'm currently on--the only one they offer in my zip code (49418). Speed tests indicate that 25 is actually pretty close to the average speed I get. However, xfinity says I can get 25, 100, 200, 400, 600, or 1,000 Mbps in my area. Is this really true, and why the difference?

I'm in the process of switching to xfinity from AT&T Uverse (which is supposedly moving toward upgraded fiber--but I've been waiting for that for years, so I gave up), but require an xfinity tech to come out next week. I was initially excited about my new 400 Mbps plan, but now I'm getting a little nervous about it, since the xfinity gateway isn't working with any of my outlets, but my AT&T one is working fine--at  only 25 Mbps. What happens if you switch everything over and I only get 25, although I'm paying for 400 plan, with a one-year contract, I might add?