The polls were accurate in forecasting a big primary victory.
You are aware that 1 out of every 7 Virginia residents live in Fairfax County? If you add the rest of Northern Virginia's population, then 2 out of every 7 Virginia residents live in Northern Virginia. That is about 28% of the state's population.
In addition, the turnout percentage in Alexandria and Arlington are very high in general elections, so polls should heavily weigh Northern Virginia.
PLUS, the pollsters always claim there are more dems than there really are, and fewer reps than there are, and way too many independents.