The difference between importing grid files using the Import as uniform lattice and Import as curvilinear lattice option is the Z value assignment. The Import as uniform lattice option assigns the Z value of the grid to the C component value and assigns the Z component a value of 0 in the Voxler lattice. The Import as curvilinear lattice option assigns the Z value of the Surfer grid to the Z and C component values of the Voxler lattice.
The HeightFields Tutorial in the Help illustrates this nicely.
Updated November 23, 2015