One thing that I have not seen mentioned on this thread is heredity. You can be lucky enough to have a slow heart rate in comparison to other people with the same relative (pun intended) fitness level.
Polar will also tell you that you can have a slow heart rate at the same effort level (hard to get your heart rate up) due to over training. And, your heart rate can be abnormally high due to over training. Kind of a catch 22.
I was told in high school that I had a naturally slow heart getting a physical for sports. When I was skiing every day I had a nurse take my pulse 3 times to get it to 44 because she could not believe her watch. In the later stages of training for a marathon I had my HR monitor on before an evening run (I hate running in the morning) sitting on the couch and noticed it read 28. I've seen 32 many times with moderate training.