Earlier quoted context omitted.
You might have failed to consider conservation of energy here. The laptops use a steady amount of energy whether plugged in or not (more or less), and if they're unplugged for a while, then they'll draw more power during the time they are plugged in as they recharge their batteries. In fact, if the batteries are not perfectly efficient and thus don't provide the same energy that was used to charge them, you're now us…
Actually, in OS X my laptop will use less power when on battery power. The clock speed goes down, the display dims, the hard drive goes to sleep faster, the display adapter switches to the less powerful one.
That's a good point; it might well outweigh the battery inefficiency's increase in power use.