Allen couldn't get a hang of Arachne's constant switching from a child to an adult and vice versa.
The way she looked and spoke - they're giving him the impression that she's indeed an adult with the body of a toddler.
Perhaps, it's more fitting to say that there's no such thing as age in her lexicon. Either she's pretending to not have interpreted his intimidation or she's just this naive. Then again, her very existence is convulated from the beginning.
"Let me get this straight. By grown-up talks, you mean to say that you've been eavesdropping around here all this time? Don't you know that's a bad thing to do? What a naughty girl," Allen commented apathetically, earning a titter from her.
"Oh, don't worry, Mister Champion~ I'm just about to get naughtier."
While he only mustered the face of a ravening predator at her proclamation, Arachne was quick to respond with the wry smile of the scheming villainess that she was.