Difference between antenna resistance and impedance
At resonance the imaginary part is zero, yes. Why is it surprising that there's still a real part?

Imagine a series (or parallel) RLC circuit. At the resonant frequency, the L and C impedance are equal and opposite. But you still have the R.

Another thing - the resistance that's left is not just radiation resistance, some of it is loss resistance - real resistive loss in the wires, etc.