How Does the Furnace's Experience System Works?

I recently saw an XP Farm working with furnaces that burn cactus and use bamboo as fuel, but I always thought that the quantity of xp given depends on how many items you collect from the "melted" item slot. After seeing this video, I realised that the melted items travel through some hoppers, and finally the player collects only a few items, maybe 1 or 2 after locking the furnaces with a lever (I also thought that furnaces can't be locked, and hoppers are the ones that can) and he gets like 7 xp levels instantly.