What makes you write that? They're a bit more sophisticated than that. The precincts are from all over the state. They would never just exit poll Northern Virginia. Last time I read, they have 19 precincts around Virginia that are bellwethers for their area, from the shore to the mountains. From the far southwest to Tidewater to the far northwest.
You realize the networks spend millions on exit polls, headed by professional statisticians and you wonder if they just exit poll Northern Virginia?