This is an example of the omnipotence of the Supreme Personality of Godhead. Although He is one without a second, He expands Himself by svamsa, His personal expansion, and sometimes by vibhinnamsa, or His separated expansion. Lord Visnu herein agrees to send His personal expansion as the son of Merudevi, the wife of Maharaja Nabhi, who is the son of Agnidhra. The rtvijah, the priests, knew that God is one, yet they prayed for the Supreme Lord to become the son of Maharaja Nabhi to let the world know that the Absolute Truth, the Supreme Personality of Godhead, is one without a second. When He incarnates, He expands Himself in different potencies.