Maharaja Prthu was well conversant in transcendental science, yet he presented himself before the Kumaras as one ignorant of it. The idea is that even if a person is very exalted and knows everything, before his superior he should present questions. For instance, although Arjuna knew all the transcendental science, he questioned Krsna as if he did not know. Similarly, Prthu Maharaja knew everything, but he presented himself before the Kumaras as if he did not know anything. The idea is that questions by exalted persons put before the Supreme Personality of Godhead or His devotees are meant for the benefit of the general people. Therefore sometimes great personalities put themselves in that position and inquire from a higher authority because they are always thinking of the benefit of others.