The file download was a bad example. I understand the file download progress bar to mean how much of the file I have downloaded. No more, no less. Next to that progress bar is the current rate and an estimate of the remaining time. This is not what frustrates me. What frustrates me is a progress bar for an installation task that sits at 99% or 100% for about the same amount of time it took to get there in the first place. I'm skeptical that this kind of problem happens merely because the last task happens to be unpredictably long every single time.

I'm betting most of the problems are less about variance in execution time and more about measuring the right tasks at the right granularity.