"I really do expect the progress bar to reflect what's going on in the background. It's kind of the point of having it in the first place - if it's wrong, why have it at all?"
Because it soothes the users into thinking something useful and important is happening when the reality is the bloody thing is just cleaning up some old files or waiting for a server connection or something equally as boring. And it's a lot less confusing for them than having console messages scrolling by.