Come to think of it, it is of course equally weird that in just about any MMORPG, using just about any weapon or spell, you *always* need between 3 to 6 hits to kill a mob of your level. This has to do with how long designers think a combat should be, and with the traditional use of random numbers in role-playing combat: A combat decided by a single "roll" of the dice would be very unpredictable. Rolling several dice produces a Gaussian normal distribution, where average results are common, and extreme results are rare.
Other than fantasy role-playing games, there are relatively few video games having people hack at each other with swords. And then most other sword-fighting games also use combat based on several hits to achieve a kill, although I'm pretty certain that in real life you wouldn't survive being hit once with a sword or an axe. But there are quite a lot of shooter games in which one-shot kills (headshots) are common enough. More importantly in those shooter games you usually have quite some distance between you and your opponent. Exchange of fire is depicted as a series of misses followed by one lethal hit. So this is what we expect, what we see in the movies, what we played in first-person shooters. Therefore shooting at an opponent at point blank range, hitting him in the head, but not killing him, is unexpected. Even in the Star Wars universe, where jedi can deflect blaster shots with a lightsaber.