For me there are two factors
1) is the stars, like we 're watching a 5* team vs a 7* team (40% more)
2) is the quality of the players.
For lv4, the 5* player has q39. For lv6 , the 5* player has q49 (about 25% more)
As we 're leveling up in higher levels, the % of quality decreasing.
Both counts for the simulation.
Another factor it's the lv.
From my experience, playing vs a higher lv/q team, I see that even if I have the ball possession and maybe the same number of shots, oppo has more efficiency, his GK is more effective and generally his defense eliminates more easy my F-K and C-K.
Also my players are more vulnerable in cards.
* That's the general idea, of course there are exceptions.