It is quite normal for the impedance of a speaker to vary with frequency. Some go from a low of around 3-4 ohms to a high of 70 or so and are rated as nominal 8 ohm designs.
This variation with frequency is a result of various electrical (and mechanical) resonances in the crossover and drivers. It is possible to construct a speaker whose crossover network presents a constant impedance to the amplifier but it requires additional components and design time. (read $$)
Kef did this for the 107 series (early 1980's) but most manufacturers don't bother.
Apogee Scintilla speakers had an impedance curve that dropped below 1 ohm at certain frequencies. There were only a few amplifiers capable of properly driving them.
A well designed amplifier with low output impedance should have no problems with any of the speakers in the shootout.