It may be that LimeWire is swarming the one song it is getting - LimeWire can and will download the same file from multiple sources giving faster download speeds. I /think/ that each host in a swarm download counts towards your total download limit.

Usually you can tell when a file is being swarmed as it will say "Downloading from n Hosts" where 'n' is a number. When it has finished downloading the file then it will move onto the queued files.

Mark