In a lot of respects, DNA is one of the more solid kinds of evidence one can have and count on. A given genetic marker is either there or it isn't.
Without a dimmer switch, the light is either on or off.
If the marker is there, then some ancestor contributed it to the individual's DNA. There's no if's, and's or but's about it.
One sister was listed as greater than 60% from English ancestry. The other had no English ancestry listed. Test error? Both have same mother and from appearances same father.
As someone trained in medical field, I would expect better results.