Real Madrid has about 30% chances of winning against Arsenal which might seems low, considering they are the defending champions however several factors contribute to their successes, and on the other hand Arsenals recent performance has been impressive with two wins and two draws in the last four matches, Additionally Real Madrid squad has been dealing with some issues, including a loss to Valencia in La Liga and a split focus between the copa del Rey finals which might have drained thier energy.
What makes me a little curious is why you often link the form of each team in the domestic league to the Champions League conditions when the match has not even happened yet? This makes your assumption strange because Arsenal's performance in the Premier League will not always be the same for other matches especially if the match is in the Champions League where Arsenal have never won the Champions League in their career. It seems that you are not very familiar with Real Madrid who have managed to pocket 15 Champions League trophies including the one they got last season, so the odds you mentioned in my opinion are not very reasonable for the winner of the match.