Pretty much this for me as well. Although, I don't think I've ever gotten anything but an 'A' in a math course. I chuckle when people tell me that being a good math student somehow translates to aptitude in the Sciences; every science course but Physics might as well have be a literature course for me.
I'm interested to see how grad school goes for me.
I suspect it's the structure of learning. Early in your educational career it's very important to be submissive and to follow instructions and later it becomes a lot less important. By late undergrad it was more important to be a hard worker who can grasp and solve abstract problems which was absolutely irrelevant, unless you didn't do your work/study, until mid-way through college for me.