Yes it was the period of time from the reset that was the issue. the read out is an average MPG since the time you push the reset button up to the time you look at it. If the engine is running (burning fuel) but the car is not moving , the computer registers that as 0 MPG - you burned a gallon, but went nowhere , therefore you got 0 MPG. That period of time gets averaged in with the rest.
So if you reset the computer, and got 26 MPG for 5 minutes, then stopped to let it idle (0 MPG) for 6 minutes your average would be 11.8 MPG.
That being said, there may be an efficency issue with the biodeisel based on chemistry. I don't know much about biodiesel, but I am familiar with Ethanol.
Ethanol produces around 50% less energy than gasoline, therefore it takes twice as much ethanol to move the same mass, the same distance, at the same speed. No one burns 100% ethanol, it's usually 10% ethanol, 90% gasoline- so the efficiency issue is lowered to 5%. If you get 20 MPG with pure gas, you'll get 19 with 10% ethanol.
I don't know what the difference between diesel & biodiesel is, but I would not be suprised if they are about the same as gas/ethanol, because in both cases were talking petroleum/corn. So at 50% BD you might experience a 25% hit. Again, based on my knowledge of ethanol, not biodiesel!