Why does npm flash "verb" and "sill" while installing things?

sill shows log level is silly in which almost anything is logged. verb indicates verbose log level that shows log messages a little bit less than silly but more than info. Regarding the npm official documentation, it has following log levels that are sorted from the least log messages to the most ones:

• silent
• error
• warn
• notice
• http
• timing
• info
• verbose
• silly

I believe this is referring to the silly (sill) and verbose (verb) log levels for npm install. See changelog here.

I am not quite sure how it ascertains which to use, but it is for the npm log files to enable easier debugging for developers.