Many hosts that you try to download from are firewalled. Firewalled hosts typically have address in the private IP space (10.*, 172.16.* - 172.31.*, 192.168.*). Companies and organizations often have private IP address for their computers since it is cheaper to not have buy blocks of public IP addresses and it is more secure.

You can get an idea of this effect with this Gnutella host graph from LimeWire. Instead of taking the actual numbers of hosts at face value notice the ratio of the green line is several times greater than the red line. The green line is the number of unique hosts and the red line is the number of incoming connections. If a host is firewalled it generally cannot accept incoming connections.