What's up with the Xfinity Speed test?

Whenever I do a speed test with the app--which is supposed to be testing between the head end and my modem without Internet latency--it reports that I'm getting 25-35% of what I'm paying for. Whenever I use other speed tests on the internet they report between 75% to 105% of my rated service (400 Mbs) The app speed test should be much faster.