Both of Monaco vs Barcelona and Atlanta vs arsenal are not a huge surprise as many people are portraying it. Atlanta were the Europa league champions beating last season Bundesliga winner Leverkusen. Monaco too is a second team in french league last season. Both of Atlanta and Monaco were playing at home. Manchester City vs Inter Milan was much surprising for me. But its first round of league and there are plenty of matches to be played.
On a norm, Barcelona should be able to win Monaco if not for the red card and I don't expect Monaco to defeat Barcelona with their complete squad and that was why it was a surprise to most people because that match made them lost their bet.
I am not surprised in Manchester city match against Inter because I know that Inter have a good defense and they play more of defensive game than attacking. Whereas, City plays attacking football and if you have a strong defense, you can resist their attacking pressure, City will not be able to score. This was what Inter did to draw the match. It is hard to win City without a good defense.