Am I getting the max mem bandwidth ? (benchmark pics inside)

[pfm]

This is for a Lenovo SL410 running a T5870 and 3GB PC3-8500 (2GB + 1GB).
Sandra Lite reports my memory bandwidth as only ~4GB/s and bandwidth efficiency of only 60%. I am hoping the dual channel is working (12.8GB/s), although CPU-Z does report the Channel# as "Dual".
I am attaching a couple of screen shots. Does anything jump out to your guys ? Or is my expectation of higher bandwidth incorrect ? In that case what am I not understanding ?

 

[Emulex]

it may be working for 512mb but not the full 1gb. It is not optimal. Throw a pair of match dimms in there.

 

[pfm]

What about the 1GB would not make it work as dual channel ? I understand getting a "matched pair" would be best but just for my understanding and knowledge.
I believe this laptop has 2 sockets only so there are probably two sticks in there - one 2GB and one 1GB.

 

[pfm]

Ok so I ordered a Crucial PC3-10600 2GBx2 matched pair kit and guess what ? No difference! The bandwidth is still the same - 4GB/s. Latency 6-6-6-15 and reported as ~85-90ns.
What could possibly be going on ?!