I've found that the *A RTW rules seem to vary by how you book!
UA won't let me fly through FRA (my start point), even if I'm connecting. Whereas when I started my AMS trip which my Travel Agent booked (through UA), this was not a problem!
UA have forced me into the 10 day, 3 stopover rule, applying the 24 hour definition of an international stopover!
Not griping, rules is rules!