Warrant Officers call Officer Cadets "Sir". Officer Cadets call Warrant Officers "Sir", the only difference is that they mean it!
Sorry for the flippancy, the point is that a Warrant Officer
is
an officer in the established rank structure
He holds a Warrant, in contrast to a Commission. NCO's in that regard are Officers, though Not Commissioned. In other words an "Officer" is someone holding authority over those who are not officers. Think of the two Ronnies and John Cleese. Cleese is the Commissioned Officer, Barker the Warrant Officer (or possibly NCO), Corbett "Knows his place".