I always retcon Obi Wan's line in ESB as him just meaning Yoda was one of his teachers (and technically the primary one since he raised the Jedi from birth and taught them the essentials) but recently I got to thinking it could mean something deeper... Watch Phantom Menace. Obi Wan respects and admires and even loves Qui Gon and constantly wants to impress and live up to his expectations even if he gently chides and teases him with his dry battle humor ("Negotiates were short!" "Why do I get the feeling we picked up another pathetic lifeform?" etc same as he will do to Anakin). Then look how utterly hurt Obi Wan looks when Qui Gon stands forward and proclaims before the council that he will train Anakin, basically tossing Obi Wan aside like garbage. I always saw this as a component of Obi Wan's resentment of Anakin that caused Anakin's training to be a little rocky. Obi Wan and Anakin had a bit of a sibling rivalry going on even though Obi Wan was supposed to be more of a father figure to him. Now Obi Wan and Qui Gon disagree on Anakin's training and this argument causes them to be at odds with one another and this bitterness is only resolved later on Naboo when Obi Wan apologizes and says it's not his place to judge his faith in the boy and Qui Gon proclaims he foresees Obi Wan will be a great Jedi knight. Finally Qui Gon dies and his last words to Obi Wan is nothing about Obi Wan but a wish, a dying request for Obi Wan to train the boy and he agrees and goes on to train him. Then he fails miserably. He failed his master who threw him away like poodoo when he discovered Anakin and who's dying wish was for Obi Wan to train the boy. Obi Wan messes all this up when Anakin falls to the dark side and does not fulfill his prophecized role as the chosen one. Obi Wan failed his master's final order. He then tries to make it up by training Luke in ANH...then he dies. At this point his mission to redeem his failing to Qui Gon has been terminated and he really has failed Qui Gon. Thus....he feels like Qui Gon would not regard him as his apprentice anymore due to his failure and instead proclaims Yoda as the Jedi Master who instructed him. He never earned Qui Gon's respect (in his own mind) or earned his role as his padawan, something Qui Gon brushed aside in order to train Anakin which then passed to Obi Wan, a role he utterly failed and failed to rectify with Luke because he died too soon to properly train him.