System is taking up 170GB of memory

Why is "system" taking up so much memory?

Why is the system taking so much memory? I have read a lot of threads on this problem, but I still can't solve it. I have checked the iTunes as they say and I don't know what I can do to solve this...

I am currently using macOS High Sierra (Version 10.13.3)

MacBook Pro (13-inch, 2016, Two Thunderbolt 3 ports)

Processor = 2.4 GHz Intel Core i7

Memory = 16 GB 1867 MHz LPDDR3

Graphics = Intel Iris Graphics 540 1536 MB

I took that screenshot a while ago, and suddenly it became this:

I really don't know why it keeps changing from "169.38GB" back to "133.75GB" then it changes back to "169.38GB" again... What's going on??? Please help me...