Why is my heart rate so high for being in shape?

(I know my question doesn't technically pertain to "tachycardia")My resting pulse while sitting is usually in the mids 80s. It has been like this for years. It ususally increases upon standing(10-20 bpm), but I drink a good amount of fluids and it's still high even when I'm well hydrated. I am 20 years old and slighly underweight. I used to have anorexia nervosa, but even when my weight was at its lowest my pulse didn't change. My blood pressure is normal. I don't drink, smoke, do drugs, or caffeine. The only medicine I take is birth control. I know 80 something is not a problem and is in the "normal" range, but I do cardio almost everyday! I run, bike, or do the elliptical for at least 30 mintues if not 90 most days if not every day. I also do some interval training. I just hate that my pulse is so high because I'm in excellent shape and so it should be lower. I find it embarrassing to tell other people my pulse because it looks like I'm out of shape when I'm not. Most people who exercise as my as I do are in the low 60s it seems. My doctor cant come up with an answer, but she's not concerned. I know I don't need to worry, but this drives me crazy!

[CCF HeartMD3]

An individual's heart rate is highly variable and dependent on many different things. It is for this reason that the range of normal for the heart rate is so large (60-100). While many people who are in good physical shape have low heart rates, not all do. For example, dehydration is a common reason that athlete's heart rate can be higher - which would be consistent with your heart rate increasing when you stand. Hope that this helps.