Progress bars—those little horizontal bars that fill from left to right as your laptop or phone updates—are notoriously unreliable. One moment, a progress bar might be 10% full. The next thing you know, the work is done. If a written estimate is provided (e.g., “10 minutes”), you might notice it change dramatically in an instant.
As it turns out, building accurate progress bars is extremely difficult because it's almost impossible for the computer to know how long the work will take without actually doing it.
This is the problem of software project estimation in microcosm.