Purple Section on Disk in Disk Utility?

Tried searching for this but unfortunately could not find anything. How do I get rid of this purple shaded area of my disk and restore my disk to full size? It's supposed to be 320 GBs. I attempted to delete a Bootcamp partition but it didn't seem to delete correctly. I understand that I will not have 320 GBs free, but shouldn't I have more than 297 GBs on a brand new UMBP? Correct me if I am wrong.

 

So it's reasonable to say that 20 GBs is lost right from the start? I am very aware of this issue on Windows computers, but I could have sworn my MacBook Air only had 10-15 GBs taken up when I got it. Maybe it's just an error on my part. So the purple section is nothing to worry about? I still do not understand why it is there. It is the exact same size as a 5GB partition.