Based on FAA advisory circular AC 150/5300-13A, Airport Design, when operating triple simultaneous precision instrument approaches the parallel runway separation could be reduced from 5000 feet to 4300 feet if the airport elevation is above than 1000 feet:
Triple simultaneous precision instrument approaches for airports below 1,000 feet (305 m) elevation normally require parallel runway centerline separation of 5,000 feet (1524 m) between adjacent runways. Triple simultaneous precision instrument approaches for airport elevations at and above 1,000 feet (305 m) and reduction in separation are currently under study by the FAA. In the interim, the FAA will, on a case-by-case basis, consider proposals utilizing separations down to a minimum of 4,300 feet (1311 m) where a 5,000-foot (1524 m) separation is impractical or the airport elevation is at or above 1,000 feet (305 m). Reduction of separation may require special radar, monitoring equipment, etc.
What is the theory behind the advice?