All vehicles that I have driven have always under delivered in this area. While it depends highly on driving habits, I believe I've done everything possible (including vehicle up-keep) to drive efficiently and I still cannot achieve what is advertised, often falling 20-25% short in real world testing.
Everyone that I know has experienced similar, and its not hard to do a quick google search to find forums with plenty of people who've experienced similar.
What gives? Is there a published method/specification/standard on how they conduct their estimates? Judging from what I've seen and heard from others, its not a problem with any specific auto maker, its all of them across the board.