In Episode I, Anakin offered to pilot his pod-racer in the Boonta Eve and give the winnings to the Jedi and Queen Amidala. This would help them acquire the money they needed to repair Amidala's ship. However, when Anakin made this offer he told Qui-Gon
You could make [Watto] think [the pod-racer is] yours, and you could get him to let me pilot it for you.
script from Star Wars: Episode I The Phantom Menace
Later, Qui-Gon lied to Watto and said he had acquired the pod-racer in a game of chance.
Why did Qui-Gon and Anakin need to deceive Watto about the fact that Anakin had built the pod-racer himself?
Note: the idea for this question came from http://www.moviemistakes.com/film976/questions/page3.