Question about RAM+Mobo compatibility

[ManOfDisguise]

So my dad had a Kingston DDR3 4GB Ram stick and he didnt need it so we put it in my PC, I had 2 slots open I have 2x4GB Corsair Vengeance RAM. 

 

This would make my RAM amount go from 8 GB to 12 GB. So we put it in, turn on PC, go to properties to check if it's recognized and it says the 12 GB ram of are there but only the 8 are usable(the 2x4 Corsair Vengeance RAM I had but not the RAM we put in the 4gb Kingston RAM)

 

So the reason these other 4 GB of RAM we put in aren't usable is I'm guessing because they aren't compatible with my Gigabyte motherboard?(I think the model was 990FX something dont remember)

 

Or is it some other reason?

[Presmo]

If the corsair ones are ddr3 then the sticks have different latencies and/or voltages