Because by the time the fight occurs at the graveyard a good chunk of the raid is afk already, because they were unable to pass middle and threaten IB, but because horde can be on the offensive the entire opening phase of the game (through controlling the middle), you don't need more than 5 people to stop the entire Alliance raid, a single hunter trap and a few mages will inevitably kill a lot of stragglers and/or slow down the alliance enough to force an unfavourable engagement.
Once the Alliance starts losing even an inch on AV its almost game over immediately, the map can very easily reverse itself into a bottleneck in favour of the horde, once horde has SH, everything south of it is automatically secured. Unlike if Alliance grab IB or FW.
Edit: you might notice the only time the Alliance actually beats horde is when the entire raid is stacked up and ready to fight, this can occur on SHGY or on the way south. This very much talks against your concept of "horde is just better", because generally the horde v ally fight is not balanced, since generally the alliance is very spread out along a marching route.