Why is remaining time estimation inaccurate?

Time estimation varies depending on a number of factors.
1. Disk fragmentation, this can be remedied by periodically using a disk defragmenter. One is supplied with Windows, and various other commercial disk defragmenters are available on the market.
2. Other applications accessing the hard-drive at the same time.
3. Transfer rate on different areas of the hard-drive may differ. This may or may not apply to your particular system, depending on your hard-drive manufacturer.
4. The size of the transferred files can play a significant role. Many small files will transfer far slower than few large or huge files.