Why Michiru and Airi talking about Airi's childhood is strange ?
It's not. There are many possible explanations. And all are equally good.
Pushing your favorite one as the Only Right One is not cool.
I just point out that no, Michiru don't know everything about the training like someone mention. She just answer you should thanks your grandfather AFTER Airi explain who her grandfather was and she have practise bowling. It's not remotely close as knowing everything.
Okay. It's one way to read it. Perhaps other people put more weight in other details.
Like, say, after the split everyone is shocked except Michiru (p13), Airi needs to explain her past to everyone except to Michiru (p14), Michiru asserts that "Airi's good for real" with a poise and confidence that suggests she knew it from the start (p15).
Maybe it's one thing, maybe it's the other, there's no canon exegesis. I'm fine with all the proposed explanations. No need to dismiss any of them.