Problem with ram

There is something strange going on and basically even I put two 8GB ram to make it 16GB it shows it's 16GB but the actual amount I have is only 8GB. I did test and see if one of the rams is bad by putting each ram in one of the slots and it works fine. The strange thing is that if I put one 4GB and one 8GB then it shows 12GB and has actually 12GB to use. I don't know if this new computer isn't working or so but if one slot is bad then how come when I put one 4GB and one 8GB it works fine but when I put two the same PNY 8GB rams to make it 16GB it won't work. I know that the notebook I have is capable of 16GB since it says on the specs. It seems like when it goes over 12GB it won't actually be useable but says I have 16GB on the total ram. I' am talking about the graph that is on the performance tab in task manager. It's like this basically I put 16GB and it says I only have 7.5GB since 512mb is used for the video card. But it should say 15.5GB if I had 16GB. I just got this Toshiba notebook 8 days agao.

 

I don't know how to explain it better but basically If I put two 8GB rams it shows only 7.5GB but the written number says 16GB. If I just put one 8GB then it will show 8GB and same 7.5GB that I can use. I just don't know if others had this problem on a Toshiba Satellite notebook. I thought maybe my notebooks motherboard is messed up and need to exchange it. Or maybe it's something you guys can help me fix?