The difference in number between one pair and seven pairs is based upon the clean and the unclean. He was obligated to take one pair of unclean animals. He was obligated to take seven pairs of clean animals. The reason being that the unclean would be naturally released into the wild, but the clean would be domesticated and used for sacrificial purposes as well as other pairs being used to propagate after their kind. A careful reading shows that both statements are true, and the difference is merely between the kosher and the non-kosher.