Well, if you think about it, the lowest annual wage is $0, the highest is likely in the billions (Gates and all those winners of capitalism aka assholes). Most people that consider themselves part of the middle class are those that live in the suburbs, have the white fence, 2 yellow labs, 2 kids and make, lets say $50000 annually.
Don't get me wrong. The definition of middle class to most people is very much those characteristics. This is simply due to the nature of capitalism. To believe you are have little to no power and exist in a low class bracket would be to admit you are a failure of capitalism. There is nothing wrong with this. The way Capitalism works, you have winners and losers. You can't have all winners. Is $50000 the median of 0 and 1000000000??? In some places, you'd be considered wealthy. Here, you are a speck of dust on a stain. It's all about scale quite frankly.
For example, using some stats that I just googled, one person living in a city of 500000+ and making less than $18,841 is considered living under the poverty line. They still pay taxes and all those wonder things. If that same person made $25,000, they would not be in the same class bracket. However, the difference between $18,841 and $25,000 is substantially less than $18,841 and $50,000. Compare that to someone making $400,000. They are still nowhere near the highest grossing individuals, yet are considered rather wealthy.
The true middle class, and those that hold any real power however minimal it may be, do not live in the suburbs. They live in penthouses and mansions. The highest class is relatively rare and pretty hard to define.
Hope that all makes sense for a 5 minute underground rant.