Quite simple. The Internet is like a big chain of computer networks, and like any chain, it is only as strong as it's weakest link. Just because your connection may be fast, the person SENDING you the information may have a slower connection speed, or is very likely transferring information over the Internet to people other than just you.

Common sense.

Also if you're downloading more than one file at once then of course the speed will be slower. A 200 Kbs Xfer speed will not remain if you start downloading a second file from the same server...the DATA transfer rate is not the file transfer rate. If your maximum possible transfer rate is let's say 2000 Kbs, that is the fastest you will get, it won't be 2000 Kbs for each simultaneous transfer you have going....

This is the same for the people sending you the data. If they are sending out 300 files at once to different people, of course it will be slow.