Dave Addey, an iPhone and iPad developer in the UK, wondered why he was getting so many typing errors, so he videoed the screen while he typed. That allowed him to watch the video to see which keys registered a touch, and the order in which they registered.
His analysis of the video showed that some keystrokes registered on the keyboard didn't make it into the document. For example, "i have" became "iave", which was then auto-corrected to "image" instead of "I have".
One suggestion is that Addey's finger slid off the 'missed' keys rather than being lifted, and so the action was ignored. That's the way other UI items work, but it's questionable whether it is appropriate behaviour for a simulated keyboard.
Addey's video is embedded in his original blog post.