I am just going by the datashet (
http://www.ledsupply.com/docs/xitanium-120v-drivers.pdf ) - It says that it can handle up to18 - 1 watt LED's.
I am no electrician, but I am guessing that it has something to do with the way it gets wired up. Their diagram shows 6 LEDS wired in series, and then 3 sets of those all wired together in parallel and attached to one driver. The driver puts 350mA across each of the parallel lines...
Shawn