It's not that fans are underestimating Real Madrid, it's that Manchester City have been incredibly strong for the last few seasons, they are going for broke, if you can call it that. Added to that is the fact that these two teams met last season and Real performed poorly, to put it mildly, getting thrashed, so fans remember it all the time
It was an unforgettable match for Real Madrid and this season it was like they were reunited with destiny and had to face Manchester City again. I think this could be an opportunity for revenge and maybe Manchester City won't be able to stop Real Madrid. Sometimes defeat teaches important lessons and Real Madrid realized this when last season they were knocked out by Manchester City. They have now cleaned up and will fight to win back the title they have been eating all the time.