I used Relationship Finder to find a relationship between my nephew, Gray-8340 and Schweinfurt-3 (30 generations) with the result showing that "Eilika is the 28th great grandmother of John", and a similar response results for my children or my siblings' children, however, if I insert my siblings (Gray-8336 - 9), it shows that there's no relationship.
As the first connection in the line is my siblings (or myself), I'd reasonably assume that Schweinfurt-3 would naturally be our 27th great grandmother.
Am I wrong and, if so, why?