"Memory Used" doesnt match memory used by apps in Activity Monitor

I have 8gb of ram which should be enough, but I keep getting "low memory" warnings. I check what is taking up the memory with Activity Monitor and the memory usage there doesnt add up. It says im using 6 of 8gb, but if i look at the memory tab to see what is using how much, the higher item is like 500mb, and the rest are smaller like 10mb or lower, so it doesnt total anywhere near the "used".