The Impedance is determined by the drivers, their impedance, and the Configuration/Wiring of the speakers. And to some extent by the cabinet.
Also, since manufacturers have immense buying power, they can afford to have speaker custom designed. In various speakers within the same design concept, some could be 4 ohm, some could be 8 ohm, and in the example already given, some could be 12 ohms or 14 ohms.
The ELAC UB5
could be a 2.5-way speakers, meaning that at ultra low frequencies, both the 5" Woofer and the 4" Uni-Fi speaker could be running in parallel. So, at those low frequencies below 270hz, the impedance of two 8 ohm drivers would be 4 ohms. (8/2 = 4 ohms)
Though I can't say with certainty, the ELAC UF5
floorstanding with three 5" bass drivers could use three 12 ohm drivers in parallel (12/3 = 4 ohms).
The woofer could in all other respects be essentially the same driver, but simply wound with 12 ohm voice coils.
Now I can't say that is definitely how it works, but it is a reasonable explanation of the difference in the speaker design, but the speaker still have the same 4 ohm impedance.
In other designs, two woofer of 4 ohm each could be in Series for a combined impedance of 8 ohms. Two Series speakers put out the same amount of sound as a single identical driver, but they have twice the power handling and half the excursion, which can be an advantage in the right design.
As someone else mentioned, it is possible to wire 4 speaker to have the same impedance as a single driver -
Though I don't have a diagram, you can also wire 9 speakers with a total impedance of a single driver. Using 8 ohm drivers, if we put three in series, then put three gangs of three in parallel, the result is the same as a single driver.
Three in series is 3 x 8 = 24 ohms.
Three Gangs in parallel is 24/3 = 8 ohms. The entire gang of nine is the same as a single driver.