Apps marked as compatible with iPad need a UI that's actually functional at iPad's screen size and orientation options, not a scaled-up iPhone interface with broken layouts or unusable controls.
Common with apps built iPhone-first that check the "supports iPad" box without ever testing the layout on a larger screen.
"Your app does not function as expected on iPad, per guideline 2.4.1. Elements of the interface appear cut off or unusable at this screen size."