Since there are (currently) four replies that express the same sentiment, let me just reply to everyone here, to avoid making this thread tediously long.
I admit it's possible I'm fooling myself, but it's the way a lot of people are saying -- that I just skated by on easy courses and missed out on the material in the hard courses. I actually got my As mostly in the advanced undergrad and grad classes. Most of my Bs came in my last year, when I had to go back and take all the intro classes I skipped in order to meet graduation requirements. The math department was fine with, e.g., letting me skipping undergrad abstract algebra because I'd taken graduate matrix analysis (which requires grad algebra), but the engineering department wouldn't let me skip intro e-mag even though I'd taken high powered diode lasers (which, IIRC, required grad e-mag II).
A lot of the Bs came in "easy" classes where the average grade on exams was 85-90. I never did enough problems to stop making mistakes, so I got an 85-90 on pretty much all my exams, which is only good for a B if that's average. The classes that I did really well in were the ones where professors asked questions on exams that required real original thought; the average grade on one of those exams might be something like 30 or 50, but I'd still get an 85-90 most of the time. The one way I selected courses that were "easy" was that I chose professors who made homework a small part of the grade, so I wouldn't fail just for not doing any homework.
Someone else mentioned Ramanujan. Well, I'm not a genius, and I know that. One of my high school friends got a silver in the international math Olympiad, and he didn't even think he was good at math because his little brother was a lot better (and his little brother later got a gold). I'm not even close to that, and even those guys aren't in the same league as Ramanujan, but I read pretty quickly and I have a decent memory for abstract concepts. I think whether or not people are sure the original poster is full of crap depends on who they hung out with in high school and college. Frankly, I was the dumbest of my close friends in high school, and about average among my college friends. When I look at my high school and college friends, any of them could have done this (with the exception of the guy who never made mistakes because he was slow and methodical, even on easy stuff), so it's not obvious to me that the original poster is just spouting off, even if it seems obvious to everyone else.
The math olympiad friend I mentioned above would go through the text for his classes and do every problem in the book before the class started. That took him a few of weeks. Someone mentioned that an engineering problem set takes more than an afternoon. That's probably true for a lot of people, but problem sets in engineering and math don't take much time if you know how to do them. The limiting factor is how quickly you come up with a solution, which can be pretty much instantaneous. There are some exceptions -- I remember some VLSI questions that basically amounted to doing a SPICE simulation by hand. And most labs are, unfortunately, gated by the amount of tedious work you have to do.
On the comment that my experience doesn't count because I'm an outlier, well what's P(guy makes original blog post | not an outlier) vs. P(guy makes original blog post | is an outlier)? I don't have any idea, but neither does anyone else here, and yet every other poster seems to be completely certain that the guy who wrote the original post is full of it. I'm not even saying that it's likely that the guy is credible; I'm just saying it's possible.
I can also believe that someone who's capable of doing that would fail out, because I almost did the same thing in high school. IIRC, my GPA was about 1.6 when I applied to colleges. I only managed to get into a decent school because I was lucky enough to be from a state that had a decent state school that had a policy of letting anyone in with an ACT score above some threshold (32 or 34, I forget which). And even then, I almost didn't got my admission rescinded by failing to graduate. A lot of teachers in my HS had a policy of giving people As in their class if they got a 5 on the AP exam. I got a 4 on the AP chemistry exam, and a big fat F in the class. I'd failed enough classes that I didn't have enough credits to graduate without AP Chem. The chemistry teacher didn't know that, but he was still nice enough to change my grade to a C when he saw that I got a 4. Thanks Mr. Swanson. I don't know what would have ended up happening if I failed out of high school. I spent most of my time in college in libraries, just reading. In theory, I could have done that even without being a student, but I doubt I would have.