That's one way to look at it. Another one is to consider the AI (offensively and defensively). I've seen enough soccer simulation games and developments (RoboCup, also as participant) to know that people develop certain algorithms on how to act as offense/defense. The most straigtforward explanation is that the difficulty level sets some kind of noise/uncertainty for those algorithms (you can also call it "randomness") such that with higher difficulty their chance of making mistakes decreases significantly, or say they have better knowledge/perception of the happenings (more precise knowledge -> better AI). One conclusion could be, that the offense takes less risks, and also sees that the defense stays well (which is also because of higher difficulty/AI) which could prevent them from playing a deadly pass now and then. That would make defending look easier.
For the offensive part, as your teammates are "more intelligent", you can probably play better passes, avoid "stupid runs" etc.
But of course, I'm just guessing here either, as you did. And I'm sure we could find a lot of other explanations for it, as we don't
know why this_and_that is going on
Superstar is easier? -> Great, good to know. Why? We don't know, just take it as it is and enjoy ^^