the whole idea of tuning a box to match the speaker no matter what frequency is to create resonance. When the box is tuned to within 2hz (imo best if within 2 if not spot on) it will give the speaker less of a chance to deviate from optimal performance.
Here just read this.
Quoted from wiki
Resonance
The moving system of the loudspeaker (including the cone, cone suspension, spider and the voice coil) has a certain mass and compliance. This is most commonly likened to a simple mass suspended by a spring that has a certain resonant frequency at which the system will vibrate most freely.
This frequency is known as the "free-space resonance" of the speaker and is designated by Fs. At this frequency, since the voice coil is vibrating with the maximum peak-to-peak amplitude and velocity, the back-emf generated by coil motion in a magnetic field is also at its maximum. This causes the effective electrical impedance of the speaker to be at its maximum at Fs, shown as Zmax in the graph. For frequencies just below resonance, the impedance rises rapidly as the frequency approaches Fs and is inductive in nature.
At resonance, the impedance is purely resistive and beyond it—as the impedance drops—it behaves capacitively. The impedance reaches a minimum value (Zmin) at some frequency where the behaviour is fairly (but not perfectly) resistive over some range. A speaker's rated or nominal impedance (Znom) is derived from this Zmin value (see below).
Beyond the Zmin point the impedance is again largely inductive and continues to rise gradually. The frequency Fs and the frequencies above and below it where the impedance is Zmax/√2 are important in determining the loudspeaker's T/S parameters which can be used to design a suitable enclosure for the driver, especially for low frequency drivers. Note that Fs is itself one of the T/S parameters of the loudspeaker.