The way I heard it explained to me is that because spider is both a color and a pattern mutation and pied is as well (look at a low-white pied if you don't know what I mean) that they "cancel each other out" and leave a high-white low-pattern snake.
Black Pewter het Hypo Vestris; Black Pastel Enchi Zamira; Black Pastel Cheryn; Hypo Enchi Sofia; Lesser Pastel Eren; Super Mojave ???; Piebald Mako; Fire Vin; Pastel Estelle; Spider Hanji, Ezri; Normal Angelina, John, Aradia; Mojave Joe; Anerythreustic Kenyan Sand Boa ???; German Shepherd Dog Atticus; Rats Snowman, Colette, Calliope, Eliza, ???, ???