Maybe you are right about using OD + ID + SB for calculating the defense. In your game did the fifth player have the highest OD + ID + SB? In my case using OD + SB should select Caetta (PG) while OD + ID + SB selects Conley (PF) correctly.
The player it picked for me was in fact the highest OD+ID+SB player on that lineup (would not have been highest OD+SB, though). Of course, I think the same player would have been the highest OD + ID guy as well.
Has anyone seen a case where it's picked a player starting as a guard for the man to man guy? (Not including defensive switches out of the guard position, though that would be kind of screwy in the first place). I wonder if it might instead pick the highest OD among the three non-guard players.
In any case, I think moutlinho's idea of posting this to the bugs forum may be appropriate. I tend to want to try to figure out the reason for this but I think it's something that should be addressed when engine fixes are someday addressed.