My bad for not being explicit.
Hitting command-J while in iTunes calls up this pop-up
, which allows you to add columns to and delete them from your iTunes screen.
I realize now that "Track Number" is totally unlikely to be the offender in your case, and as my best guess I'll offer up "Date Added" as the criterion that results in the segregation.
Edit: Note that adding or deleting a column does not
affect the effect its contained data may have on sorts.