Well, let me speak from an experience I had with a REx that may explain your discrepancy.
I was out test driving a service loaner for a drivetrain malfunction after the REx ran for an extended period(this was before we knew about the moisture in the REx generators), and was coming back from a trip to the beach trying to get the fault to appear. I was about 2 miles from the dealer, and was running on the REx when I ran out of gas according to the gauge-which shuts down everything, no driving power available(even with battery power still showing as ~5%!!!). Being so close to the dealer, I really did not want to jog back for a gas can, so I tried a few things trying to eek a bit more juice out of it. One of my ideas was attempting the Emissions Inspection Mode, to recharge the vehicle just enough to let me drive it up the last hill. Popped the hatch and started the test mode, and the REx fired right back up and ran without a problem for almost 10 minutes before it began to stumble and then shut off from fuel starvation. This means there was enough gas left in the tank to support high speed running for 10 minutes, but the REx would not use it in normal operating modes.
I obviously cannot prove this accounts for all the discrepancy reported in tank capacity, but it does make a difference. I would be curious to see how close to being truly empty the UK REx system will run compared to my findings. Maybe the reserve BMW incorporated is not a physical reserve, but a software modification to shut off the REx a bit sooner than the UK counterparts?