I do not claim to be an expert in the application of DNA analysis to genealogy research, which is why I always turn to the experts when questions come up. (See previous post on genetic genealogy resources.)
More than once I have been approached by someone who asks, “My brother (or sister) and I both had our DNA tested and our results are different. How can this be?”
In a recent EOGN blog post, Dick Eastman addresses the same question and points to a lengthy but coherent answer at the website called Genealogy Explained. DNA analysis is not nearly as cut-and-dried as we would like it to be. As with most topics, especially genealogy, we must look beyond the surface to get the whole picture. I encourage you to read the original post and educate yourself on the intricacies of DNA analysis as applied to genealogy.