From your question, it sounds as if your neighbor lives in a property that you assume is very similar to yours. However, if his lot is bigger or his townhouse has a bit more square footage, that may explain the difference.
In addition, fees can depend upon the location of each house, townhouse, or condo within a community. For instance, if your HOA covers lawn maintenance and your townhouse is a corner unit, you may pay a bit more than someone who lives in an interior unit with a smaller yard.