My guess is a number of adds provide TBN, including calcium and zinc. Also, I though I read that an ester base oil provides a TBN effect although not measured in the TBN test, which might explain why Redline TBN is not as high as some would expect. OTOH, maybe I don't know what I am talking about (except calcium--a no brainer TBN constituent).
Hey, I bet a search of the Question of the Day and/or Interesting Articles forums would answer the TBN question.