Guys, there is a way to reset trip computer to show you correct average......overtime due to stop n start driving trip computer reading becomes only approximate/indicative 
For everyone's benefit Here is the procedure that I use in my car(Not a Corolla but principle is same):
1) make sure your trip computer is showing average L/100Km then reset your trip computer by pressing and holding the button/knob that you use to flip through different display modes
2) Once trip computer is reset release the button/knob
3) Start driving on highway with a constany speed, dont press accelerator too much....just press enough to hold speed at 80 Km/H
4) Drive like explained in step 3 for 10 Kms
You'll see your trip computer starts showing the reading you expect from your car 