Maybe one of the channels is broken? Try it with sticks in one channel then the other to see if that gives an answer.
I've seen Tradesman say that if the cpu cooler just presses on one side of the cpu it can do something to the memorycontroller on the cpu.So check the cooler.
Did you try each ramstick on it's own.
And what about the specs for your hardware? What is your ram configuration and motherboard,maybe even cpu.
Some Hardware memory is reserved on windows platforms to my knowledge,meaning a small difference taken from your maximum ram is reserved as mb.Also some memory is cached for faster access on operating ssytem