Minecraft wiki entry for Damage, associated text: While a 23 block fall should be fatal for a player at full health (23 - 3 = 20 × 10 of damage), due to the way fall distance is calculated, a 23.5 block fall is required instead, because the last tick of fall distance isn't added in when calculating fall damage. For certain heights, including 23-block falls, this discrepancy is enough to avoid an entire 1 of damage.