You say that, but I didn't hear a rule against it with だって

Grammar Explanation: That said with だって

Standing back to back, Sacchan is slightly taller thank Yui. Yui questions whether Sacchan is standing tip-toed, which Sacchan denied, but Yui looks and sees that Sacchan is.

さっちゃん:: 「だって背(せ)のびしちゃダメなルールって聞(き)いてないし」; "You say that, but I didn't hear a rule against tiptoes."
結衣(ゆい):: 「背(せ)のびしてもいいルールなんてないもん」; "There's no way there'd be a rule allowing tiptoes!"