Chrome Version: 73.0.3636.2
OS: Win
Graphics Card: Asus Radeon RX 580 4GB

I think this is a known, but not logged issue. Also there is an existing bug for building a better test site for floor height testing (bug 909933).

What steps will reproduce the problem?
(1) In the Oculus desktop app, go to Devices > Configure Rift > Floor Position and enter your actual height. e.g. 5'11".
(2) Place the headset on a table, or sit down and place it in your lap (within view of the Oculus sensors).
(3) Open (or restart) Chrome and go to https://immersive-web.github.io/webxr-samples/positional-audio.html then press Enter VR.
(4) Pick up and put on the Oculus headset.
(5) Stand up and notice the floor height.

Expected: Floor height should always be close to the real floor height.

Actual: Floor height is very different if you enter VR with the headset at waist height vs at standing height. E.g. if you:

(2) Stand up and place the headset on your head. (you'll need to see the desktop in the next step. What works fo me is to tilt it up so it rests on my forehead) 
(3) Open (or restart) Chrome and go to https://immersive-web.github.io/webxr-samples/positional-audio.html then press Enter VR.
(4) Look through the Oculus headset and notice the floor height.

At this point the floor height is pretty accurate, and is very different than the height from the steps above. They should be the same.