It is indeed odd, but I stopped losing sleep over it (not that I actually was) when I realized that even if it were 100% accurate, the ethnicity really didn't help me with finding or confirming individuals in my family tree. I understand that many people appaer to be mainly interested in the ethnicity, and I'm sure part of that is based on how ancestry markets this service.
One think to keep in mind is they specifically say this is all "beta" meaning it's not a finished product and there will be bugs - not just in the software, but in the science behind the software. I'm not sure if the general public understands the implication of "beta".
As far as me not matching my mother, I have a whole thread about my situation where this is discussed:http://boards.ancestry.com/topics.dnaresearch.autosomal/37/m...
My conclusion was the only way that could happen is if they are looking at different things for cousins than they are for ethnic groups, and in fact they are. Even though the basic information they get from your DNA is the same, the cousin matching looks at specific aspects of it with a specific formula to find matches, and the ethnicity looks at different aspects with a different formula.