The subject matter is many times more difficult, yes. Most biology classes just involve memorization, whereas physics, engineering, and math tend to require a decent amount of memorization (albeit quite a bit less than bio) combined with a thorough conceptual understanding of the material to work through a few very complex problems.
The standards for the subjects are also very different. At UNC, passing physics and math classes was considered good work, and an A showed that you had truly mastered the material. Very few people got A's. Bio classes were a different story. I'd say that "good" range that a C constituted in physics/engineering/math was a B+ or A-. For most people, the difference is that your physics/math/engineering degree is extremely marketable; I have numerous friends who are already making $50-70k a year with only their BS's and 2 years of experience. A bio degree is just a gateway to graduate study, more or less. It should make sense that the degree that prepares you directly for a career is going to be the more rigorous one.
When you're applying to med school, taking classes that impose the "passing is good" standard is generally not smart. You want to pad your GPA as much as you possibly can, for you'll be up against tens of thousands of other people who did just that. You may have taken far more difficult classes than they did, but your 3.3 is still going to look worse than their 3.7's by quite some margin. Just something to think about.
