As to the number differences, Independents (about 10%) can't vote in the party primaries, but could vote on the initiative. As to the "undervote" that you think might mean a person went and voted on nothing (which could happen just be being really dumb), if some people voted for candidates but not the initiaitve (didn't care, or didn't understand it) and a few others voted on the initiative but not candidates (independents per above, plus a lot of folk in both parties who favored "None of the Above") you could easily have individual race totals well under the total number of voters and yet everbody voted on SOMETHING. Keep digging!
There have been several accounts of voter fraud in Florida.