Well we all know the answer to that. 3 reasons - 1) differing amounts of effort. 2) Our guys were always leaking out before a defensive play was completed. Nobody does that on the other end (hmm we have not yet scored, I better start running back to protect the basket!). And 3) Pace. If the nature of your fast break points is from guys leaking out and just pushing the pace, as opposed to say creating TOs then you are naturally going to give up a good number of fast break points the other way because your defense isn't set either.
In fact, 6 of the top 10 teams (Denver, Houston, Atlanta, Sac, OKC, DAL, CHA, LAC, GS, Utah) in fast break pts/gm place bottom 11 in fast break points allowed/ game, of which only Houston and GS were playoff teams and very well known for their run and gun styles. The other 4 were all playoff teams and conversely were in the top 11 in fast break points allowed/ game. Of these 4 teams, 3 were in the top 10 in opp TO/ game. We were 15th in that category, notably highest of the 6 teams that were in the top 10 scored, bottom 11 allowed categories.
My take away from this? Defense first, and if that opens up fast break opportunities then take it. Run and gun rarely works.