Just to add, yes you inherit 50% of your DNA from each parent, and matching to relatives by descent involves identifying segments in both your and your matches DNA which are identical. Beyond smaller segment lengths, the results are indisputable.
Ethnicity matching involves identifying segments in your DNA which the testing company consider similar or identical to patterns found in the DNA of other individuals (not necessarily related to you by descent) who they have allocated as originating from or being most often found in a particular region. The results and regions differ between testing companies according to their algorithms, reference population samples (which can be as few as 20 or so people in some cases) and whatever else they decide to match on, or not. They (the testing companies) will all have different characteristics that they infer ethnicity matches from, and different opinions of the geographical areas that constitute their reference regions. At continental levels, it can be considered reasonably accurate, but at more local levels and with the smaller percentages of your DNA that they might allocate to them, it may have no actual basis in truth. In other words, it is as much of a black art as a science, although they will try to persuade you otherwise, and best taken with a large pinch of salt!
It's the eye-catching "promise" that draws in customers, many of whom have little interest in finding relatives and most recent common ancestors. That's all, really.
