Why did the mileage for Audi A3 decrease from model year 200

Why did the mileage for Audi A3 decrease from model year 2007 to 2008?
Aren't they using the same engine?

Listed mileage for 2008 A3 is 21/29
For 2007 it is 25/34

Anyone have any explanation?

This is true for most cars. In 2008 a new procedure can be used to estimate mileage figures. This new approach is closer to 'real world' mileage numbers than the last estimates.

The EPA changed the way they measure mileage. The old system used slower speeds, slower acceleration and no A/C. Everybody complained that they could not get the EPA mileage so they came up with a new more correct system. The mileage holders get will be exactly the same.

From Edmonds:
WASHINGTON, D.C. — The U.S. Environmental Protection Agency posted new fuel economy ratings for vehicles from the 2007 model year and earlier using new calculations it says are more realistic and reflect real world mileage.

As anticipated, many bigger vehicles, like the Hummer H3, also as gas electric hybrid vehicles, are seeing mileage declines under the new system. The 2007 H3 four wheel drive model with an auto transmission gets new numbers of 14 mpg in the city and 18 mpg on the highway versus 15 mpg in the city and 19 mpg on the highway under the old system. Toyota's Prius hybrid saw a dramatic decline, too. Under the new system, the 2007 Prius gets 48 mpg in city driving and 45 mpg on the highway. earlier, the EPA said the Prius returned 60 mpg in city driving and 51 mpg on the highway. Toyota noted that mileage ratings for competing vehicles will drop, too.

Tables that list the new ratings compared with those for last years may be found at fueleconomy.gov. Window stickers reflecting the new fuel economy tests will be obliged on 2008 models.

The EPA explains that its new system — the 1st change since 1984 — factors in such variables as higher speed and greater use of air conditioning.

What that means to you: effectively, your vehicle's mileage has not really changed. What changed is the way the feds compute it.