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.