As per my understanding 6509 all slots are dual channel, so 9 slot * 40 per slot (20 g in and 20 g out) = 360 GB
How cisco claim the 720 ?? What about the 6513 chassic switch fabric connection?
As I told above all the line cards and Supervisors in particular are different in FABRIC ASIC sub-modules. Your 10g line card has 2 fabric ASICs thus can use both fabric channels. Sup720 has only single fabric ASIC and can use only one channel.