RAM usage never more than 80%?

Okay, so I'm playing watch dogs which needs 6GB ram on a 4GB ram PC.

I run compact tray meter and one thing I notice is that the ram usage never goes more than 80%. In fact, whenever it loads stuff from the HDD ( I could see HDD usage 100% and stuttering in my game), RAM usage decrease to 77-78%. VRAM is kind of healthy at only 52% of 2GB used.

I want to know why not all 100% of my RAM is used? Hardware reserve is only 145MB.

And why is the minimum system requirements for watch dogs a GTX460? I mean, I got a 645M, I can play it, my friends with a 640 and 630m can play it fine, our GPU temperature is just 80"C even after hours of playing.

What you are seeing is normal. The system memory manager always maintains control over RAM usage, the goal always being to maintain the best overall system performance. 100% memory usage as shown by the gauge is a bad thing and the system would permit this only with the most extreme workload, which the game is not. Understand that the memory gauge does not show all of memory usage. It was never designed to. Typically almost all of RAM will be doing something.

Memory management in Windows is extremely complex with many factors involved. It is the result of more than 20 years of research, development, and testing. Even an explanation of the basic concepts would take considerable time.