Track your mileage by fillups instead of trusting the computer. It usually reads a bit high, but it can be calibrated and perhaps it's calibrated wrong.
In any case, diesels don't excel at stop and go traffic. And of course a long hill climb is going to result in poor mileage, but a long downhill will result in great mileage.
I'd track a few tanks and take notice of what your general driving conditions were. I use
www.fuelly.com