As Timh correctly noted, impedance and resistance are two different measurements. Impedance (Z) is made up of Resistance ® and Reactance. Your meter is measuring resistance so it will read less than the impedance of the circuitry.

So how does this relate to speakers, amps and impedance matching?

Most speakers are 8 ohms (many car audio ones are 4) and so amp manufacturers typically size their amps to efficiently drive 8 ohm loads. When you start wiring multiple speakers in parallel to a single amp channel you begin to lower the effective impedance that the amp sees. You can see this by the simple formula that 1/Ztotal = 1/Z1 + 1/Z2 + 1/Zn. If I wire two 8 ohm speakers in parallel, my equivalent impedance is 1/Ztotal = 1/8 + 1/8...Solve for Ztotal and you get 4 ohms. Some amps are good down to 4 ohms, some only to 6. Check your amp specs to see.

So, what happens to your amp at these different loads. In very oversimplification and using ohm's law for pure resistance, current = Square Root (Power/Resistance). If we lower the impedance (resistance) for a constant power (our amp), the current increases. More current = more heat and our amp bakes. If we raise the impedance, the current decreases. Since the current drives the magnets on the speakers we end up with less ability to efficiently drive our speakers. So, in essence, our amp really wants to operate in that sweet spot of 8 ohms.

Now, if you want to connect multiple speakers to a single channel on an amp, we must find a way to get the equivalent impedance back to 8 ohms. As you saw, if we wire two sets in parallel we have 4 ohms. But if we could add a device to the circuit that multiplied the impedance of each speaker by 2, then we get back to 8 ohms. We see this by 1/Ztotal = 1/2x8 + 1/2x8 = 1/16 + 1/16 = 2/16 = 1/8 = 8. Back to 8 ohms and all is good. That's where impedance matching volume controls or switches come in handy. And it is why you will see them often with dip switch or jumper settings labled 2X, 4X, 8X, etc. It means 2 (times), 4 (times), etc.

Another example, if we have four sets of speakers in parallel, then we connect them to four VCs, set them to 4X and end up with 1/Z = 1/32 + 1/32 + 1/32 + 1/32 = 8. Where it starts to get a little tricky is if you have 3 or 5 speakers. Most VCs/Switches don't have 3X or 5X settings. So we have to compromise. For 3 sets, we could use 4X and end up with 1/32 + 1/32 + 1/32 ~ 10 ohms. If we used 2X for the same scenario we get 3/16 ~ 5.3 ohms. In this case 10 ohms is better unless you KNOW your amp is good down to 4 ohms. At 10 ohms, yes our speakers are a little less efficient but we aren't burning up our amp and you'll probably never notice the difference.

I hope that helps a little. I really oversimplified alot of the engineering, but hope you get the point.