First and foremost, you are sharing the server with other people. Also, internet congestion and receiving end speed affects how much you can transfer upstream.
It's very rare to get full link speed outbound. There's a lot of factors causing this, too long of a list which can affect the transfer speeds to list here.
Somethings affecting are is the torrent a fast & busy one, or just a few dialup users downloading it? If it's a private site, these are often really hard to seed. Public trackers are easy seeds due to the inherently different seeder to leecher ratio, less seeders vs. leechers as no one is trying to maintain atleast 1.0 ratio.
If the receiving end is on the other side of the globe, it's also unlikely all links up to there would be unsaturated etc.
