Default number of rows displayed for "SET FEEDBACK"

A colleague walked into my office and asked me what the default threshold was for the number of rows in SET FEEDBACK. I said "I am pretty sure it is just on", but wanting to be absolutely positive, I looked it up.

SET FEED[BACK] {6 | n | ON | OFF}

How odd! Why would the default be set to 6?? What kind of number is that? Why would you have a default number at all?

The colleague walked out with a smile.