Many progress bars or other indicators lie, and the incentive is always to make it look good at the beginning, so that’s what we end up seeing most, whether it’s these ad ones (which thankfully I’ve never seen) or installers or especially something like Uber that always lies about how quickly someone is coming to make it appealing and then stretches it out. Even the thing in your car that tells you how much range you have left before refuelling (except it starts showing more than you actually have). I think in all cases it’s probably possible to give a more realistic estimate but it’s counter to the goals of whoever designed it.