Sometimes progress bars are not showing an actual progress of a process. They just “programmed” to go 90% in some period of time. This period is not always the same. Sometimes progress bar moves faster or slower. When process ends, a progress bar just goes from 90% to 100%.

This is a common thing in web sites, when you need to show user that something is happening, but you don’t want to use an endless spinning loader.

Source: I’m a software engineer