And I often do exactly the same thing--in fact more often than not. But I have also hunted moose enough to know they often take a while to tip over, even when hit perfectly through the chest. It simply takes a while for their system to shut down, and shooting them again doesn't have much if any effect. In fact it can get them started going again, probably though adrenalin.
The same thing is true of some other large animals. It's exactly why African PH's wait a while before following a Cape buffalo even after it's taken a solid chest hit. Waiting gives the buffalo's enormous lungs time to fill with blood and his systems to shut down. If you follow one up and start shooting immediately, adrenalin kicks in and the buffalo becomes much harder to kill.
