Because of Gnutella's size, the nodes you connect to is fairly "random". It's not truely random, because there's always a chance multiple nodes connect to the same node. But apparently, in your case this didn't happen when you tested it.

So, when your PC connected to a number of nodes, they may have had connections to other nodes who had plenty of the files you were looking for. On the other hand, by pure chance, your MacOS connected to a number of nodes that didn't share much (of what you were looking for). Its partially a design flaw (because this possibility CAN be reduced a bit), but its also a "user fault" (as in: users leeching instead of sharing).

Since the MacOS and PC version of Limewire have exactly the same Gnutella cores, there's otherwise no difference - except for performance based on the connection of each computer, and the system configuration.