Well, it was a state college, I'm actually really good, and we couldn't pick graduate classes in the US, even though I was a senior already, and if you compare standards that basically means graduate. But most likely it just wasn't an engineering school.
I've also had other classes, though (languages, marketing ...), and those were really easy as well.
What did you study? Either you're exceptional, you've never been to an engineering school in the US, or you were taking particularly easy classes.