Zulu casualties

The of Isandlwana has always interested me, but one thing i have been wondering about recently has been the apparently few casualties on the zulu side. Most experts seem to place the Zulu casualty figures around the 1,000 - 1,500 area, which is roughly the same as on the British.
So we are saying that each british soldier took on average one Zulu with him. That seems strangely low, as the battle lasted about 4 hours, from roughly 11am to 3pm, and for much of the time the British firing line held the Zulus at bay. Each British soldier apparently carried about 70 rounds of ammo, and it is suggested many used them all up, and may have been resupplied. So roughly 1,000 soldiers discharged 70 rounds each, so where did all those 70,000 bullets go ??
In addition the two field guns were heard firing up until about 3pm, you would have thought they alone would have killed many hundred in massed ranks of Zulus.
Even after the camp fell there were several last stands, including a large one of perhaps 150 soldiers who formed a square below the mountain. They would surely have accounted for several Zulus each.
Clearly my figures are very rough, but you can see my point. Anyone have any ideas ??