Nav2k's correct, I'd just like to add that having dual, low impedence voice coils allows for a very large number of wiring configurations.
When dealing with normal speakers, a higher impedence is better for sound quality. This doesn't matter with subwoofers as the sound isn't distinctive like midrange is.
Lower resistance places higher load on the amp because current increases, higher current at a given voltage equates to a higher thermal dissipation which is a consequence of the amp producing more power.