To return to Joe's original insight -- why are some notes hard to
match singing? -- I have the same problem (and pretty much of a
tin ear; electronic tuners are a blessing), except reeds are for
me the best instruments. I tried singing to match a note from
a midi script, using the many instrument voices available, and
the organ was the easiest. The theory that the harmonics match
one's voice sounds good to me.

I also find that when I try to learn a song from a recording,
some people's voices are easier or harder to match. I've found
that Garnet Rogers sounds great, but I can't sing along and
find the tune.