DNA testing companies and fabricated African ancestry

How reliable are DNA test results?
Snopes investigated whether DNA testing companies deliberately fabricated African ancestry to "screw with" racist customers as claimed by "a number of less than reputable web sites" pointing to DNA testing companies such as Ancestry.com and 23AndMe.
It found the claim was "mostly false" and that "DNA testing companies strongly denied any such practice. The claim was traced to "one anonymous worker and published on a humor site; even the worker indicated purported meddling was unusual."
Suspicion of results is understandable. My AncestryDNA shows 19% Great Britain which is a best estimate with a range of  0%—47%. One of my brothers might have 0%, another 45% and all would be entirely consistent.