I have to imagine this is a result of program culture and how the county draws up the district lines. Even though both schools have, say, 1200 students to draw from, that doesn't mean they have the same number of students who are interested in playing football.
A similar discrepancy exists between Sherando and James Wood, two Frederick County teams. James Wood has only beaten Sherando a few times over a 30 year period. Wood has gotten a couple wins lately though I would put that down more to Sherando declining than JW suddenly putting things together given the lack of success the Frederick County schools have had at the regional and state level in recent years. But despite that the two have had a ton of close games. Has EV dominated Culpeper on the scoreboard too or just in outright W/L?