Can't
That couldn't happen... Dialups are limited at 53 kb/s under absolutely ideal conditions (living inside your ISP's office). This means that 53/8 = 6.625 KB/s is the fastest you'd be able to average a file download speed, not 11.67. Your software could have been confused and miscalculated the first bits that came in and therefore upped the average, but the best way if you really wanted to figure it out is to watch your network activity, and set a stopwatch, then divide your file size by your timed download results, it cannot ever exceed 6.625 KB/s.