When a speaker is labeled 8 ohms, it is actually a nominal reading.  An ohm meter measures the resistance to direct current (DC), but music is alternating current (AC).  A speaker's impedance to AC depends on frequency and the nominal rated impedance is sort of an average impedance over the useable frequency range of the speaker.  You will find that a 8 ohm speaker will be between 5.1 and 8 ohms, 16 ohm speaker will be between 11 and 16 ohms, and a 4 ohm speaker will be between 2.1 and 4 ohms. 

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