The important thing is what impedance load they present top the amp, DC resistance is irrelevant. The impedance will change with frequency as it is a combination of mainly capacitance and inductance effect of resisting AC signals. Think of it as an electrical spring, a capacitor is like using a tight spring, and an inductor a loose one - the former poses less 'resistance' at higher frequencies (as ultimately no current flows across the plates, only charge) whereas an inductor increases resistance as the current frequency is increased.
A loudspeaker driver is effectively a large inductor with the mechanical spring effects of the driver, plus any capacitors in the cross overs, cable effect etc all influence the electrical resonance frequencies. The impedance drops where these 'resonances' occur