The key factors in growing premium wine grapes are the microclimate and the soil. They impact the ripening and flavors, and define the characteristics of a wine. The microclimates and soils in Sonoma County are different from those in Napa, even though the two counties are side-by-side. Winemaking is also key to the properties of the finished wine, but we can assume equipment and practices are similar.
So it's the microclimates and soils that make the wines different. Let's take a deeper dive into why the grapes coming into the winery can be different in these two neighbors.