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I'm thinking it could be dependant on the amount of voltage being supplied to the speaker by the amp at the time of measurement. So it may vary with how one is measureing. I could be wrong though (probably am as electrical is not my cup of tea). I'm sure someone with more knowledge will chime in shortly. Do their measurement data lend any clues as to how they got their numbers?

Ohm's are a unit of measure for Impedance: Impedance = voltage/current
So, how do you tell what the impedance of a speaker is? On most cabinets, it should be printed on a label next to the jack. If the speaker is visible, it may be printed on the speaker label or stamped on the frame or magnet. To measure the true impedance of a speaker or cabinet requires a rather complex procedure involving signal generators, power amplifiers and high frequency AC voltmeters. However, with raw speakers and many cabinets, the ohmmeter function of a digital multimeter can help you identify what the impedance of the speaker should be. Generally, the reading given by an ohmmeter will be about 2/3 to 3/4 of the impedance of the speaker. So, a 4 ohm speaker will typically measure about 2.5 - 3 ohms, and an 8 ohm speaker will typically read about 5-6 ohms, while a 16 ohm speaker will measure around 12 ohms.
So they are both more-or-less correct!
 

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Speaker manufacturers are supposed to follow rules for measuring and labeling.
The impedance is measured using AC (a multimeter measures resistance using DC) and the readings do fluctuate.
So if the impedance readings fluctuate, what number are you supposed to go by?...
This is two parts...
1)The nominal (or average) reading.
AND
2) The minimum reading.

A manufacturer may rate the speaker using a single number, which is usually the nominal reading (i.e. 8 ohms) but this nominal reading can only be used if the minimum reading never dips below a certain percentage of the nominal reading (off hand I can't remember what this percentage is). As an example, a speaker may be rated as 8 ohm if the nominal reading is 8 ohm AND the readings never dip below 4 ohms. But if the minimum reading does dip below 4 ohms, the speaker is not supposed to be rated as 8 ohm, the rating must be adjusted accordingly (lowered).
Many speaker manufacturers cheat this rating (possibly SVS?). A higher and a more consistent impedance reading is easier for an amp to run. This is why Jon Lane proudly proclaims and labels his Chane speakers as being TRUE 8 ohm rating (or in the case of the A5rx-c, it is TRUE 6 ohm rating).
 
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