Your ISP provides yoiu with 1024kb/s download and 256kb/s upload.
Bit torrent protocall doesn’t measure in kb/s (1000 bits/s)
it measures in kB/s (1000 Bytes/s)
20kB/s is equal to 160kb/s
Now, if your client is properly tuned, you should be able to get adverage download approaching your upload limit, or about 30kB/s, and with torrents that are extremely well seeded, you may see much more.