300 ft altitude change
Think of airplanes coming the other direction on a adjacent track at the same altitude, or at neighboring altitudes. If you both have to deviate toword each other. The guy deviating to the north of his track will descend 300 feet, the plane deviationg south will climb 300 feet, giving you up to 400 feet between you.
If you both changed kept the same altitude, you could collide. If you both changed altitudes by 500 feet, then you risk collision from adjacent tracks and adjacent altitudes doing the same and all ending up in the same place.