How do you define class in America? As an expatriate Englishman, the notion of 'class' has a different flavour to me. In popular terms it is perfectly possible to be poor but upper-class in the UK (there is a charity called 'the Society for Distressed Gentlefolk') or rich but 'working-class' like professional footballers. The British Civil service has an elaborate definition of class based on the kind of employment (Civil servents of couse being in the top class). This is not to say that you can't move between classes, but that there are strong cultural elements to each calsses self-definition.
How is this defined in the US? Here the notion seems to be far more closely allied to the amount of money earned, so statistics like you quote may well be self-fulfilling prophecies.