I'll try to offer some detailed insight on my behalf. Your mileage is shockingly low, and it sounds like you may just need the firmware update to get your trip computer calculating correctly.
2010 CT EX-L w/ Navi & 4WD - 26,500 miles
-I bought this vehicle with 17,500 miles on it. At this time, I had Trip A & Trip B reset. I reset Trip A every time I fill up. Trip B gets reset every oil change.
-Just had the oil changed today, Trip B reported 25.4MPG.
-Filled up before driving 200 miles home yesterday, Trip A reported 30.8MPG. (Note, Western PA to Eastern PA is slightly downhill most of the way.
-I drive 22 miles each way to work, 21 of this is 65MPH/55MPH 4-Lane Interstate.
Based on simple hand calcs to check the trip computer, the mileage the CT is reporting appears to be a little generous in my case. When my Trip A reports, say, 26MPG, my calculations usually come to about 24.5MPG. Etc. Etc.
I rarely do more "around town" driving than "highway" driving, but the few times I did, my mileage reported closer to 21-22MPG.
I was only getting 28-30MPG in my Civic Si, so I'm happy with the mileage I'm seeing out of the CT.