It really isn't again.
Middle income is generally going to be defined as something more specific and significantly lower than everyone who arguably can be defined as "middle class."
The term middle income tends to be defined as follows.
http://dictionary.reference.com/browse/middle-income
Given the US median household income is around 52,000$, claiming that someone making around 250,000$ is middle income comes across as awfully dubious.
People in some cases might argue they are "upper middle-class" which extends how high income can arguably go in this category considerably. Now you can argue that "middle-class" in the U.S. is generally too broad a term and worthless, but Romney specifically used a term which generally people would certainly not find remotely accurate to apply to people making 250,000$ per year.