The bar moves in steps, so heights cluster
USATF youth meets raise the bar in 5cm steps, just under two inches. High school meets usually go up two inches early and one inch near the end. Nobody clears 4'11" because the bar was never at 4'11". A jumper capable of 4'11" gets recorded at 4'10", and the average you read online inherits that rounding. Pole vault has the same rounding problem, visible in average pole vault height for high school.

