Obviously, the idea of Yoda's other pupils hadn't been explored during production of the OT, and it was thought that he was Obi Wan's sole teacher.
Episode II established Yoda as being an early teacher with the dead meat younglings scene. Padawans apparently move on to a single jedi master as they grow up.
Dooku went Sith much later in life than Anakin. And Qui Gon was his padawan...