That's that nice Nacra quality. I got some new rudder arms for my Nacra 20 and one was 4 inches longer than the other one! 4 inches! Christ, that's not just a little mistake. I measured them with my existing ones and luckily the longer one was the wrong size and just a simple cut was all that was needed.
Do like Dave said and cut your tiller arms a little and re-drill.