The Google backlink algorithm is probably guarded just like their search algo, and I have yet to see a decent explanation of how it works. On average G shows about 5% of the backlinks Y does for my sites, and when I take the time to look through the backlinks I find that I've got no BL showing for some good, high PR link trades but some do show from free sites that have me with some small/new LLs.
Would love to hear a good explanation.