How can multiple woofers generate lower frequency?

As Marty and John have explained, the extra drivers mean you can play those frequencies at louder volumes. Cabinet size is also a factor, and the taller cabinets have more internal volume.

I'd also be very suspicious of any speaker that claimed usable bass at 28Hz from a single 6.5-inch woofer. Barnyard epithets come to mind.

There are several reasons to use several small drivers rather than one large one. First, it allows you to build narrower cabinets, which the women-folk appreciate. (Two 6.5-inch drivers can probably move about the same amount of air as one 9-incher.)

Also, multiple drivers means drivers at different heights, with different floor-bounce arrival times. Floor bounce can cause suckouts somewhere in the midbass range. Multiple drivers will have different suckouts--so you get more of them, but none as deep as if you had a single driver.

Not necessarily. Multiple drivers ameliorate one problem, but there are other, and better, ways to deal with even that one. (One is to put the woofer near the floor; that way the suckout frequency falls well above the woofer's range.)

And how much that matters depends on the room you're in. In fact, given the room problem, there is no one design that's better than the others. Which is why it's a good idea to audition any speaker in your room before you buy (or lose your return privileges).