Intel Motherboards : ASrock Z75 Pro3 - Strange Memory Problem

I have strange problem with my motherboard (ASRock Z75 Pro3). I have 4 identical memory stick - 4 x 4Gb Patriot 1866 DDR3. If I populated all 4 memory slot, the BIOS only show 8Gb (A1 and B1 are set as empty in BIOS) at only 1333Mhz. The weird part is if I select System Browser in main screen BIOS, I can see all 4 DIMM at the correct speed (1866Mhz). 

If I remove all memory stick and put only two memory stick in slots A1 and B1, BIOS show 8Gb of RAM so those two memory slots are functionning, it's not a hardware problem.

I can't find a way to correct the problem

I have the lastest BIOS update (2.00) and try couple of clear CMOS with no result.

Thank for your help