It's not always the fault of the device. It estimate the time based on the download speed in this moment, but it can't know that in the next moment your ISP server slows down or that your AV stops the download for some times to analyze a suspicios file, things like this to make your day enjoyable. Because of this, never automatic updates. I recieve an notification if there is an update in the queue and so I can update when I like in a spare time.