Because some of the newer boards allow it. I'm mainly talking about the older ones like Socket A.
Even some of the newer boards will only let you fill up two slots inwhich you'd need 2x2gb sticks which are expensive as all hell.
However, sometimes in the older systems instead of using 2x512 and putting it in 1&3 people have been able to get DC by putting in 2x256 in slots 1&2 which are channel A and a 512mb in the 3rd slot which is Channel B. Same thing applies to those newer boards, instead of having two 2gigabyte sticks, you can put 1gb sticks in Slots 1 & 2 (or which ever colors match) for Channel A and then matching 1gb sticks in 2 & 4 which has the same principle as the 2x256 in the 1 & 2 on the older boards....make sense?
Well just a wild guess what since it is blanked out this means there isn't daul channel memory?
PS. What is the advantage of dual memory?
Hmm, you have 1 or two sticks that make up that 1gb? Really shouldn't matter though should say Single...weird...unless for whatever reason CPU-Z doesn't like anything below PC3200 but that'd be weird.
Perhaps your mobo or CPU simply does not support dual channel.
The advantage of Dual Channel isn't insane so I wouldn't get too worried about it. Anywhere from 5%-20% gain in performance. Obviously 20% would be nice, but it's not always that.