On Wireless Power Consortium webpage there is an article about Magnetic Resonance. These paragraphs in the page surprised me:
You might expect that operating tightly coupled coils at resonance offers the best performance. That combination, however, is not used in practice because two tightly coupled coils cannot be both in resonance at the same time. This is one of the counter-intuitive effects that make power electronics such an interesting subject.
Most Qi transmitters use tight coupling between coils. In that configuration, the best results are achieved by operating the transmitter at a frequency that is slightly different from the resonance frequency of the Qi receiver. Off-resonance operation gets you the highest amount of power at the best efficiency.
I couldn't understand why tightly coupled coils operating at resonance do not work with best efficiency and instead they should work at a frequency slightly different than resonant frequency.