The muliviscosity oil are based only on their viscosity at 212 Deg. F. They are formulated to meet the W-grade designation of a realively thin oil , at a particular low temperature.
They meet the standards of the thicker non W grade oil at higher temperature, usually at 212 Deg. F. Even though the multiviscosity oil has a thinner base stock, the oil is tickned with additives. That my explain a little better why the multiweight oil flows ticker.