Graduate classes usually require more individual and critical thinking. So you not only have to have a firm grasp on the material, you also have to be able to apply it very well. At least, that's how it is with the classes I took back when I was going for a PhD! (All the test questions would give an experimental scenario with data and ask you to explain it, and then ask you to design more experiments to test your explanation, etc. And the tests lasted for like 5+ hours, during which I would sit there combing my brain for appropriate experimental procedures!)
The other thing is that in grad school you just have to make a B to pass, but your grade doesn't matter AT ALL other than that. So when I was in grad school (one semester) I got a B, B+, and A-, and thought I was doing great. Until I dropped out to apply to med school and realized that I have to report my grad GPA to AMCAS 😛
eh, it doesn't matter. A few Bs aren't going to kill you. Is the class at least good/educational?