Hmmm. Interesting that Heathrow's mobile site doesn't seem aligned with what their main site (
Heathrow: Flight information | Status updates) shows. That would certainly explain what we're seeing.
Out of curiosity, I've just installed Heathrow's Android app - that currently shows "AIRBORNE: DEPARTED 09:21" for EK008 (which is more or less correct, it was airborne at around 09:20:50). The main site also shows "AIRBORNE 09:21".
So it would seem that the mobile site wrongly associates "AIRBORNE" with the aircraft starting to move ("DEPARTED" on its own would be technically correct), and then the airborne time gets updated when the flight actually takes off.
As they say, go figure.
