Seems like an error on your ISP part. To fix this, contact your ISP and either:
1. They cap the down/up speed to the values as specified in your contract.
2. They ask you more money for much faster speeds than specified in your contract.
Of course, there also can be a Speed Boost feature that you don't know about. Some ISPs (including mine) do offer Speed Boost for slower connections to ease the browsing, where anything that's downloaded is smaller than e.g 10MB in size, the Speed Boost activates and downloads it (usually) 3 times faster than the regular download speed specified in the contract.
Since most torrents come with packages no bigger than 4MB, you can use the benefit of Speed Boost for the full torrent when there are plenty...