Intel Motherboards : Memory Slot and Memory Usage

I have Asrock z87 Extreme 4, Intel i5 4690K 3.5 GHZ and Team Dark 2x4GB DDR3 1600 memory installed. The Bios and Windows recognizes memory as 8GB too. 

My question is, as the CPU Usage is 70% - 100%, The memory doesn't use more than 60% of the total amount of the memory installed. Is it normal? Why the memory seems hard to use more than 60% of the total amount while available. 

To make it clear, i'm not sure if the memory usage ever reached more than 60% before, but, before i create this thread, i tested to run heavy programs (FSX, graphic editing software, chrome, etc) to see if the system run fully load. After several minutes, the maximum memory usage is only about 5.1GB of 8GB installed.

I re-checked again to the Asrock mainboard product page, at the Memory support list section, at the top of page it says: 

Note3: For 2pcs (DS), please use A2 and B2 slot.

I currently use A1 and B1 slot for the memory. Is it probably the issue? What is the different and the function of those slots?

My setup:

CPU: Intel i5 4690K 3.5ghz

Mainboard: Asrock z87 Extreme 4

Memory: Team Dark DDR3 1600 2x4GB

Graphic Adapter: Zotac GT440

OS: Win 7 64 Sp1

This is, of course, not the Mainboard or hardware related issue. I just wanted to know how my setup could run effectively.

Could somebody please help me to increase the memory usage... 

Thank you in advance, Regards.



Edited by wardog to remove the garbage at the end of the post