Beasts get to roll two dice, pick the higher (not 'highest' when there's only two - who taught GW writers grammar anyway??) and double the result for moving through difficult terrain. 'Nids get the Move Through Cover rule, so 'Nids with the Beasts rule get to roll 3d6, (adding one die - singular for 'dice' = 'die'; GW and good grammar don't mix) pick the highest and double the result.
Leaping provides no advantage when moving through difficult terrain.
Hormogaunts benefit from all of those rules - fleet, all troop members within 3" during an assault get to attack, 3d6 through difficult terrain, pick the highest and double it and 12" assault range. Only the assault range is redundant.