I am not even sure I would say “usually”. Rashbam and Rabbeinu Tam often differ with their grandfather Rashi. And Rambam was in a direct line of talmidim from the Rif, yet often disagreed.
Compare how similar the baalei tosafos are with each other, as opposed to with the Chachmei Sefared, for instance, and vice versa. Also, you’ll never find a talmid always agreeing with his rebbe, and those differences will be magnified in each subsequent generation, especially when there were multiple rebbeim. Still, the relative similarity still speaks of an oral tradition.and