1) Please read
this sticky2) Some of your upload speed is also being used just to run mIRC, as mIRC has to upload and download information from the IRC server(s) for normal chatting, and the status window.
3) Your upload speed will also be affected by any other programs that are using your bandwidth.
4) The rate that you download is restricted by the senders upload available bandwidth as well as their available download bandwidth.
There may be other factors in play also.