Yeah... if you're doing more than one thing on the net at once, the bandwidth gets divided.
So if you have a 1mbps cap and you're downloading a file at 100kB/s (about 800ish kbps) you only have about 25kB/s (200ish kbps) left for anything else you might be doing. Browsing is usually given priority over downloading too, and the bandwidth is usually divided evenly between downloads or applications, but that's not always the case.