Someone from another site has suggested that I might inadvertently disconnected the kindle from the cover.
http://the-digital-reader.com/2016/0...-case-battery/
He is right. This might happen when I read on the sofa and the device briefly slipped from my hands before I caught it. So, I did two more run of my experiment but this time I made sure the device never slipped from my hands.
The result is much better. It lasted 11.5 hours, light level 24, WIFI off.
It lasted 11.5 hours whether I read 11.5 hours straight through or with interruption in between.
I will do another experiment. This time with the light at level 10. We will see whether the Oasis lasted as long as it does according to Amazon's claim which is 28 hours.