8 gigs should be 9 gigs of ram, no?

i notice a lot of off the shelf "i" systems sell 6 and 8 gig ram systems. Isn't there something with tri channel ram where you should have like 6, 9, 12 gigs. This has just been gnawing at my brain for a while and thought someone could splain it to me. Thanks. 

Memory sticks are made in 512MB, 1GB, 2GB, and 4GB sizes.

The 500, 600, 700, and 800 series Core "i" systems all run with dual channel memory configuration.  Systems with dual channel memory have either 2 or 4 memory slots.  You must either install 2 or 4 memory sticks to be in dual channel operation. 8GB is a possible config here (Two 4GB sticks, or four 2GB sticks)

The 900 series Core i7 runs with triple channel memory configuration.  Systems with triple channel memory have either 3 or 6 memory slots.  You must either install 3 or 6 sticks to be in triple channel operation.
6GB is a possible config here (Three 2GB sticks, or six 1GB sticks - although the latter is less likely)
Some motherboards allow 4 sticks to run in triple channel operation, and so 8GB is possible as four 2GB sticks.