This because an ohm meter is measuring the DC resistance of the coil windings.
Impedance is similar to resistance except impedance measurements are for AC signals. Anything music is going to be an AC signal.
Also, the listed impedance for a cab or speaker is the "nominal impedance", nominal meaning, in name only. The actual and instantaneous speaker impedance is dependent on what frequencies are applied to it at that instant. This impedance can change over a relatively large range.
For connecting speakers/cabs to amps, you only need to concern yourself with the nominal impedance of the entire cabinet. In most cases this will usually be 4 Ohms or 8 Ohms.
If you suspect that someone has changed some of the speakers, you may need to measure each one individually. To be sure they are all the same, per the @Stewie ohm meter method. Individual speakers can be measured like cabs, but to do it requires disconnecting each speaker so it is not influenced by the others. Then you need to be sure you hooked everything back up EXACTLY as it was or you run into other problems. All of this may be getting into unfamiliar territory, and that's not bad.
Unfortunately, this mystery cab ended up with someone with little experience in these things. That's just going to put you in a very steep learning curve, early on. A guitar player can get away with a lot more variations or unknowns in the speaker/cab department. But when it comes to bass, as @agedhorse says. You better know what's going on in there. You could damage an amp.
You might be better off having a qualified tech check out this cabinet for you. Be sure to let them know you intend to use it for bass guitar.
Other than that... Welcome to TB, even though it seems you jumped right into the deep end. It gets easier with time and there are lots of good folks here who can help guide you in the right direction.
