Okay,Consider this scenario..You are playing a 4-4-2 and your opposition is also playing a 4-4-2.Now your team is considerably weaker than the opponent.
Consider these OI's
RB- to tightly mark LM.
In the OI-
LM- Closing down always,Tight marking always.
LB -Closing down always, Tight marking-no instructions.
So according to the OI the RB will stick like glue to LM.BUT in the OI you have also set the LB's closing down to always.So the RB will still ALWAYS close down on the LB if he is within range, even though he has been ordered to man mark the LM.
It happened to me once,My RB was marking the opps LM,but he quickly closed down on the LB(who was in possession of the ball) once he was in his (closing down ) range.The LB managed a pass to the LM who had plenty of space to put a cross in and i conceded a goal..