I have always believed this to be true, which is why I think authors should read for just a couple of minutes, and the rest of the time, answer questions if they don't know what else to say. Staying attentive while listening to someone read is hard!
I once followed along with the author while she read the first chapter - it was only about 85% the same. An author's impulse to rewrite never goes away, I guess.
Someone at Powells told me they once told an author he would have about 45 minutes. He said "Great - I have a passage that will take about that long to read."
Honestly, I've been to two readings in the past week, and as much as I really did enjoy hearing both authors read their work, what I most enjoyed were their stories about the stories behind their books, or about their writing processes.
Lisa Madigan had a teenage boy read her first book - it worked really well. And it was short.
I like your readings because you tell about your experiences, funny little encounters and things you've done in the name of research. Makes for an interesting time. I do like the reading part, too, but I agree that it's nicer when it's short, just a little taste.