Harrodian said in the admissions email they took different education backgrounds into account so i assume that means state versus prep as they use ISEB which the preps do prep for and state schools don't.
I think the out there interview questions are probably more to see how you think, can you think on your feet, change tack, defend an argument, use critical thinking. You cant prepare for questions like that so I imagine quite a valuable tool for schools! DD loved the Harrodian interview as it was just so wacky and out there.