Because it's got a big list of n-grams and knows what words are more commonly used than others. It uses a predictive model to generate a list of words from your

Because it's got a big list of n-grams and knows what words are more commonly used than others. It uses a predictive model to generate a list of words from your swipe, and then sorts that according to usage. It also continuously learns what words you use the most and prioritizes those. It picks the most likely word, but also displays a list of other possibilities you can tap. It's pretty slick.