The Z68 platform is engineered to use dual channel memory. If you only have a single slot populated it will be running in single channel mode, which cuts the bandwidth in half. When you run WEI it will see that it is not getting the throughput that it should have for that platform, and give you a low score because of that.
I am using "Zappelin 4GB Single Channel 1333 DDR3 (1x4GB) Motherboard is Gigabyte Z68AP-D3
My problem is the Windows Experience index of my desktop pc RAM
Windows Experience index gives 5.9 to ram and 7.5 to cpu but on my "vaio F23EFX" RAM index is 7.5 for both of 4Gb 1333 Ram and i72670QM CPU
According to the Windows website, if your computer has a 64-bit central processing unit (CPU) and 4 gigabytes (GB) or less random access memory (RAM), then the Memory (RAM) subscore for your computer will have a maximum of 5.9.
This might explain things, since your desktop pc might be 64-bit.