Reds vs. Pirates..appeal play

[rbmartin]

Runners on 2nd & 3rd. 2 outs. Batter lines clean single to left. R3 (Griffey Jr.) easily scores. R2(Dunn) also scores easily. Following the play Pittsburgh appeals that R2 (Dunn) had missed 3rd base . 3rd base Ump calls out (replays were inconclusive but to the umps credit he was right on the play looking at the bag as the runner passed so I assume the call was correct).

After a brief conference, not only did they wipe Dunns run off the board, they removed Griffey's run as well.

The batter didn't even get credit for a base hit. I think officially it went as a Fielders' Choice.

I don't know if it mattered but Griffey had not crossed the plate by the time that Dunn passed (apparently without touching) 3rd base.

a) Could someone explain why the lead runner did not count.

b) Had the lead runner crossed the plate first, would it have made a difference?