It doesn't recognize you singing because it doesn't have the same spectral content as the full song. Shaman doesn't really identify tunes, it identifies recordings. (And it is amazing to me that it works at all)
For people who are interested in these things, I'd highly recommend Chris Okasaki's thesis/book on the topic.
I was not familiar at all with this stuff when I read through it the first time, so it was a tad mind-bending and I probably understood ~10% of it, but it was certainly educational.
What are you referring to here?
It doesn't recognize you singing because it doesn't have the same spectral content as the full song. Shaman doesn't really identify tunes, it identifies recordings. (And it is amazing to me that it works at all)