What's considered a "missing piece"? 0

[Black Bat]

I collect primarily golden age, and I've noticed that on some CGC books graded 6.5 and 6.0, there's pieces missing like small 1/8 -1/4" triangles from corners and 1/4" chips from edges.  When I look at the Overstreet grading guide, it says missing pieces are not allowed for these grades.  In fact, missing pieces aren't allowed at all until 4.5.
So why do these books receive higher grades from CGC?
Are these defects not considered missing pieces?  
Does CGC use their own grading guidelines?  
Thanks!

Edited June 17, 2018 by Frank Grow
added pics

[AJD]

Yes, CGC uses its own grading standards, and they clearly differ from Overstreet in many points of detail. They are not published and the only way you can get a feel for them is by experience.

 

BTW, nice book! Overstreet and CGC would both agree that's it nicer than my copy. 

[Not A Clone]

Golden Age comics in particular seem to get a grading curve "bump". Check out the "Can you spare a grade?" section and look at some of the grading contest comics for some examples. Overstreet is still a nice place to start. 

Edited June 19, 2018 by Not A Clone