I only get about 81 or 82Mbps on an iPhone 5s on a 119Mbps Comcast connection, using the latest Airport Extreme. I can get 113Mbps with iMac consistently, using speedtest.net. I can get 105 or so with an iPhone 5. Not sure why the iPhone 5s consistently gets less speed. Perhaps differences in the Wi-Fi hardware, but it is hard to believe that. Antenna? I don't know what it is, but it is consistent (no matter how to position the phone related to the router, or holding, or not holding the phone, etc. using the same speedtest.net app in both phones, both phones running updated software, etc.). Tried multiple times, this is not a one time thing. This is perhaps either a hardware or driver issue. 
Now, let me get into conspiracy theories, perhaps Apple is not offering over 100Mbps in their Apple Store Wi-Fi so people cannot easily notice these differences. It would be hard to believe they haven't seen this. This is using their own router.