What's odd about that ?
Clearly, some people have only one arm. It's caused by birth defects, accidents, medical trauma .. a variety of possibilities. We're all likely to have met someone with less than two arms.
In many statistics, such deviations from the norm are balanced against similar deviations on the other side. There are people under 6 feet in height but there are also people over that.
But there are vanishingly few people with more than two arms. Perhaps a tiny number of birth defects. No accidents. No medical issues (Zaphod Beeblebrox is fictional). So they won't balance out the numbers with less than two arms, and the average (the mean, that-is) is below two.
Statisticians know that mean averages can sometimes obscure the truth and so they have other averaging methods to choose from. The median average (middle of the distribution) will still be less than two but perhaps higher than the mean. Modal average (the most common occurrence) will be a solid two.
Yes, it is very common for the inappropriate method to be used to calculate such an average. Often intentionally, in order to mislead the reader - hence the phrase 'lies, damned lies and statistics'.