Might have to do with fact that when Kobe is out, the Lakers' bench is playing against the other team's bench and the Laker's bench includes Nick Young, who would get starter minutes on many teams in the league. Ed Davis' shot blocking with the second unit has also been pretty good.It has to be more than just that. When Kobe is on the bench the Lakers give as good as they get, or so the numbers seem to say, but when he's on the court they're terrible.
The first team has had it's problems on defense and that includes Kobe. Also, first teamers, Jeremy Lin and Wesley Johnson's play has ranged from inconsequential to terrible.