Income is one of those measures that varies greatly across families. As the chart below shows with the green bars, small families of only two people tend to have lower median family incomes than other families.
Of course families of two might be two young people just starting their careers, so their income earnings tend to be lower than well established workers. On the other hand, two person families also tend to be people age 65 and older - people who may well be living on retirement incomes which are often significantly lower than what they earned during their prime working years.
Looking at the red line in the graph, you can see that the most common size family are exactly this group. Nearly half (49%) of Herkimer County families are comprised of two people. About a quarter more families (24%) are made up of three members. Only 4% of all families have 6 or more members living together.