While reading Kare Kano, I noticed that the extras included in the tankobon compilations seem to have absolutely no relation to the story of the main manga. (I have just finished the fourth volume of Kare Kano, so I've read both "The Tiger and the Chameleon: A Promise for One Week" and "Meet Me Again Tomorrow in the Forest". The former involves students in high school and has a romance theme, but it does not seem to belong to the same setting as Kare Kano. The latter is a story of a half-alien, half-human girl's experiences with racism on another planet.)
This observation was backed-up by some research that I did: the two extras that I have read so far appear to be works written by Masumi Tsuda in 1994 and 1993 respectively, prior to her publication of Kare Kano.
Why were pre-existing unrelated works included as extras? In the past, had other manga compilations done the same thing? I could imagine this as a means to "promote" the author or provide readers with bonus content at low cost, but in other manga that I've read, such as Fullmetal Alchemist, extra one-shots were clearly related to the main content.