There are a number of potential causes. Upload bandwidth limiting is one. You also have to share the available bandwidth with all the other people uploading from that client. And there's all the search and ping/pong traffic moving in and out of many clients.

I would also take the speed ratings with several grains of salt. I'm on a cable modem but have my speed set to "T1" because LimeWire makes some assumptions about a "cable modem" connection that are not valid for my particular cable modem. My cable modem easily supports connections to at least a dozen other clients, but LimeWire won't let me specify anywhere near that many if I have it set to "cable modem". So I lie and say I have a T1. I'm probably not alone in this regard.

Also, keep in mind you're downloading from another person's computer, not from some expensive web server. You're unlikely to ever see the kind of transfer rates you get from good quality web sites.