You might find it a little odd, after pages of adventure, despair, and hope, to encounter a sort of Japanese comedy duo at the end. However, the two investigators ask Pi some important questions and, more importantly, act as liaisons between the doubtful reader and the text.
We're not sure when you jumped ship, but Martel increasingly tests the limits of his readers' faith. Maybe you grimace before you even begin and say, "A boy and a tiger in a lifeboat? Like that could ever happen." Maybe, as Pi's survival extends to an unprecedented 227 days, and he hones his skills as a shark-thrower and hawksbill connoisseur, you say, "Enough's enough. I want realism." Most readers probably raise The Eyebrow of Disbelief when Pi meets another castaway on the Pacific Ocean and discovers an island made entirely of seaweed. The Japanese investigators are right there with you. They tell Pi flat out: "We don't believe your story" (3.99.1).
Their admission gives Pi a chance to defend his tale. He links storytelling with faith. He talks about how our understanding of the world shapes the facts we share about it. He explains the danger of reason on its own. And he expresses disappointment in the investigators' expectations. (He believes they want a "a story they already know.") On a theoretical level, Pi defends himself well. But the knockout punch happens when Pi tells an alternate version of his story. He retells the shipwreck, his survival, and his 227 days at sea without the animals. In their place, he puts himself, a Taiwanese sailor, his mother, and a cook. The story is horrific. Even ghastly.
Now for the BIG question: Which version do you believe? Do you think Pi, as young boy, comes up with fantastical tale to cope with an ugly truth? Or is it somehow not the point to decide what actually happened? That the beauty of the first story outweighs the believability of the second? On the one hand, Martel spends a good 200 pages developing the first story and about 7 on the second. The sheer volume, the proliferation of details, favors the first. On the other hand, the first story is also totally unlikely. We're not going to tell you which story to believe. Uncomfortable? Good.