Eleventy - Lesson 22: gorko _config.scss

Dear Andy (or other helpful people),

sorry if this is a dumb question, but what is the logic for labeling $gorko-size-scale 300/400/500 etc.? How relates this to the Fourth scale?

It would also help me if you could explain the reason for choosing the breakpoints at 37em and 62em. Or is this explained somewhere in the Lessons?

Thank you very much in advance
Harald