Thanks, Herb!
Both RealTracks and Vocaloid use segments of "real" audio, but Vocaloid is much more granular - imagine trying to splice a saxophone phrase not on a beat segment, but on the sustained portion.
And then auto-tuning the heck out of it.

They're working on the problem with expression, but it's complex.
In the mean time, I've got to spend more time working on learning to sing properly.