That's because even Flick decides to rotate 80% of his first 11, they are still stronger than Osasuna. What bookies look out for mainly is team strength, team depth, previous meetings that's H2H and sometimes team names. For instance, we all know that Manchester City is deep in form from the Premier League but even if they meet a team like Nottingham Forest that's above them, there's every chance that Manchester City will still get a smaller odd compared to Nottingham Forest.
You can't compare Nottingham vs City and Barcelona vs Osasuna because Osasuna is much stronger and it will be a match between the favourites and the underdog, otherwise it will be an even match between teams from the top of the table. And Flick won't change 80% of the starting lineup to see if they can beat Osasuna in this way, I don't even think they can, look at how the first match between these teams in La Liga this season ended.
Osasuna is much stronger than who exactly?? Did you even get the point as to why and how I brought in Manchester City Vs Nottingham Forest?? You don't make references? Or you don't give stances? It was boldly written, "for instance" Maybe you actually do not get the point I was trying to make. Yes Flick won't rotate his first 11 that's why I said "even if"