frequency - Natural and Resonance frequencies of a damped oscillator

The damped oscillator equation is

\begin{equation} m\ddot{x}+b\dot{x}+kx=0 \end{equation}

And its solution has natural frequency $\omega_0$

\begin{equation} \omega_0=\sqrt{\frac{k}{m}-(\frac{b}{2m})^2} \end{equation}

However, when one adds a driving force to the equation

\begin{equation} m\ddot{x}+b\dot{x}+kx=D\cos(\Omega t + \phi) \end{equation}

the resonance frequency $\Omega=\omega_R$ that maximizes amplitude is

\begin{equation} \omega_R=\sqrt{\frac{k}{m}-2(\frac{b}{2m})^2} \end{equation}

I'm wondering why the resonance frequency isn't the natural frequency. I've read this formulas in the wikipedia page of the harmonic oscillator.