Synastry/Composite - Contradictions?

I'm confused -
So let's say you have Sun square Merc with your partner in synastry but sun conjunct or even trine merc with your partner in composite, how does that work out? Because in synastry, people say that partciular aspect can cause arguements but then you have the composite saying communication is swimmingly?

Your communication works in synastry but not in composite - I don't get it? OR You have no Sun square Moon in synastry but Sun conjunct moon in composite?
 
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