Tristram's like the Wizard of Oz, pulling strings and making his marionettes (i.e. family) dance. By now you shouldn't be surprised that, even though the book is called The Life and Opinions of Tristram Shandy, Tristram isn't a central narrator. He tells everyone else's story: his parents' marriage, Uncle Toby's love affair, and Slawkenbergius's opinions on noses. He's also got a little too much info for his own good—like what his parents were doing when they conceived him. He's "indebted for the preceding anecdote" to his uncle" (1.3.1), but come on: how does he know this stuff?
And there are lots of non-Tristram voices clamoring to be heard. We've got long passages from sermons, books, and even rites of excommunication, as well as stories from the always-entertaining Trim and Toby. Is Tristram insecure about his own voice? He quotes his mother's marriage settlement because the point "is so much more fully expressed in the deed itself, than ever I can pretend to do it, that it would be barbarity to take it out of the lawyer's hand" (1.15.1).
When he begins Slawkenbergius's tale, he generously says that "they are to be looked upon by the learned as a detail of so many independent facts, all of them turning round somehow or other upon the main hinges of his subject, and collected by him with great fidelity" (3.42.2). Both times, he gives up many pages of his own story to allow someone else to speak, and both times he praises the other writer. You might say that Tristram is a very generous narrator. You also might say that he's got an agenda.