In short, its your internet.
First of all an 8meg line is not 8 megaBYTES its 8 megaBITS. so its not as fast as it sounds. Plus when they say 8mb, they mean that in the best case senerio, you may get 8mb however they don't promise a thing.
I also have a 8megabit line and I'm able to get about 700kilobytes per second. However, I'm the nearest house to the internet hub so that why.
Last of all, I wouldn't be worried about what the speed test says, what is more important is how fast you can download a file from a site.