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 usable 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.