Why does it seem like I have less RAM than advertised? Why is that on my Galaxy Note 3 (3gb RAM) I only have .91 GB free, 1.5 GB used while doing nothing, meaning only 2.41 GB total. Is that because the rest (~.6 GB) of my memory is used for the OS? And the reason I only have .91 GB free when doing nothing isn't because 1.5 GB is cached because I cleared my cache and the number did not change. 

Why does it seem like ~0.6 GB of RAM is missing?
3Gb advertised by the manufacturer is the total capacity of memory chips installed in the device. But this memory is shared between multiple components (like AP, BP, GPU, etc). 2.4Gb reported by android is the amount available to AP. The rest is not "missing". It's just being used by other components.