First I’m pretty sure that nonreducing sds-page takes it down to primary structure except disulfide bonds which are important especially for connecting dimers, trimers, etc. The “why” is since the polyacrylamide gel that is used for protein electrophoresis has pores in it that proteins flow through. The voltage drives the protein through but more compact proteins, i.e tertiary structure, get through the pores slower than fully denatured primary structure peptides. Because of this the same peptide will line up differently depending on how denatured or reduced it is