This junk once made me download the entire game (12 GB) again for a update and it took the entire day to download.
it's the games fault. all game libraries download data in a non ordered system byte by byte and part of what takes downloading it so long is that it has to decompress and spend extra processing speed locating the next chunk if data to install. Then it has to actually write the data. If it downloads again that probably means that it either has two parts to the download or your first Installation was corrupted
Steam does this too all the time with updates. This is why you'll often download 30mb worth of stuff and then get zero kb for a few seconds. That wait is your stuffty write speed and the compressed game's confusing architecture. And if you quit while it's writing it will unwrite all of the data