Whenever you are looking at an average of something, it's useful to ask yourself, "plus or minus what?" Averages nearly always have uncertainty associated with them because they are calculated based on a sample of a larger population.  For instance, if you want to know how happy people in the U.S. are, on average, you could try to figure it out by asking them, "On a scale from 0 to 100, how happy are you?" But of course, you can't pose that question to everyone in the U.S., so instead, ...