Because even though he was birth in Mexico, he was still an American citizen because his parents were.... So even though he wasn’t natural born, he was still able to confer citizenship to MR, who is natural born. Rubio, however is NOT natural born and NOT eligible
They claimed to have retained their U.S. citizenship, but no one has any evidence to document the fact that they never surrendered or forfeited their U.S. citizenship after emigrating to Mexico and immigrating back into the United States as refugees from the Mexican revolutionaries. So far, all we have are unsubstantiated claims by the Romney family and their supporters.
Furthermore, they disregard the past practice of the U.S. Government advising U.S. military families that their foreign born children of U.S. citizen are not natural born citizens. This was done by officers of the U.S. Government in the 1945-1980 era before ignoring the eligibility issue became popular.
Thanks, DM, makes sense.