You're right, Skip. Page 21 of the Manual says the DTE initialises at 5 litres remaining. There no reference to its deriving its rate from an average from the last fill. The context in the Manual implies the DTE is based on the consumption rate at the time of initialisation, which is confirmed by your and my observation.
Another aspect that has been alluded to quite a long time ago when this subject was last beaten around the scrub, is the unreliability of the DTE kms after the ignition has been turned on and off a couple of times. For that reason, I try to pounce on the Trip A reset as soon as the left segment starts flashing, and work on 65 km available around town regardless of the DTE's read-out. Uninterrupted, the DTE is quite reliable. I use a spreadsheet and allow 1 litre unusable (it's actually 0.8 litre) and compute the distance remaining at refill using the fuel added to a standard fill point. The computed DTE is within a couple kms of the indicated DTE or Trip meter figures.