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.