Just an objective question and certainly not meaning to stick up for NHRA.....but wouldn't "household income" include everyone in the household? (i.e.: husband & wife)
Perhaps as a marketing ploy, they use "household" to jack up the numbers for the sake of advertisers instead of "individual" readers. At first glance, it would look like their readers would have more money to spend with their advertisers. Somehow I don't think that most wives would be part of the target audience most ads are for.
But Beard is right. The board of directors would likely up the average quite a bit!