Vasudeva was in the complete perfection of life in considering Lord Krishna and Balarama to be his sons, but because the sages assembled in the place of pilgrimage at Kurukshetra had spoken about the Lord as the supreme cause of everything, Vasudeva simply repeated it out of his love for Krishna and Balarama. Lord Krishna did not wish to detract from His relationship with Vasudeva as father and son; therefore in the very beginning of His reply He accepted the fact that He is the eternal son of Vasudeva and that Vasudeva is the eternal father of Krishna.
The Power of Vasudevas Sons
His Divine Grace