Do loading bars freeze on purpose?

Many applications implement some sort of progress bar or spinning circle to indicate loading of significant size.

In Windows 10 updates and some games, I've noticed that these loading elements sometimes actually freeze and then continue. However, this really doesn't make sense since I'd imagine the UI is running asynchronously as otherwise the UI would always be frozen.

In the case, since the UI is most likely running in an asynchronous thread, is the freezing done on purpose? As a method of telling the user that something is actually happening?