The numbers are "best guess" using epidemiological modeling. Not all people who get the flu are tested or treated either. Over time, these numbers will accumulate so we'll have a better resprsentation of what is going on.
I'm not getting hung up on whether the mortality is 3.4, 0.3 or 0.01. We've got a population of 350 million or so and 30-60% will probably get this in some form from inapparent to life threatening. The number that will require extensive medical intervention will overwhelm our health care system if they all get sick at the same time.