Bukhari wrote: "I have not included in my book al-Jami` but what is authentic, and I left out among the authentic for fear of [excessive] length.(Footnote 2)"
Footnote 2 says:
He [al-Bukhari] meant that he did not mention all the turuq [parallel chains of transmission] for each and every hadith.[7]
http://www.islamic-awareness.org/Hadith/bukhari.html