Your mileage will also improve after the car is broken in. That happens after 5000 mi. or so. I suspect in your case, it's mostly the short trips. Short trips kill mileage in any hybrid. You're still getting at least 30% better mileage than you would in a non-hybrid under the same conditions.

Winter is also a big factor. The anti-Prius (see below) can get as high as 12 l/100 km in the summer on the highway, but in winter in the city it burns 17-18 l/100 km. That's a big range. Yes, it's not a hybrid, but use the comparison.

Pearl is a
2007 Driftwood Pearl Prius
Package "B" (everything but leather, nav, and rear camera)