If this was a zombie apocalypse a la Walking Dead, I would die after 72% of the population. But the excessively fast, axe-wielding, simultaneous barrel-throwing zombies of the House of the Dead series (where Typing of the Dead came from) would kill me instantly.
You're not mumbling anything. And the random words is so that you actually read and comprehend what you're having to type instead of predicting the next word based on sentence structure. Otherwise you'll see "I'm going" and your mind and fingers are already prepared to type "to" or any other manner of words that could logically follow that phrase.