Headcanon: When he hits his teenage years, Aang starts fucking with his friends by making up all sorts of wild stuff that totally happened 100 years ago. Toph always says "he's not lying", but at this point the Gaang knows that The Gremlin Supports Lying For Fun, so Aang always suggests going to Bumi to confirm whatever bullshit he's spewing this time. And he always does.
The Gaang figures that Aang must be somehow coordinating this with Bumi, because no matter how absurd the lie is, Bumi is always 100% on board and can elaborate on it seamlessly. They keep trying to find out how Aang and Bumi are coordinating the lies, going so far as to intercept Aang's letters to Bumi to check for secret codes, but they can never find anything.
And the truth is that they're not coordinating. Bumi just rolls with whatever Aang comes up with because it's hilarious and Aang is his buddy.