I used to know a doctor who told his patients something like normal was a guy in Arizona who ran 5 miles a day. "Normal" by most definitions I think just reflects statistics. Most of us have been taught along the way that a normal temperature is 98.6 but if you take 100 peoples temp I bet you are going to see quite a variant. Of course it depends on where you take the temp.
The whole point of this is the most important thing and that is deviation from the norm and how the patient feels. When I was 18 my normal blood pressure was 80-90/60. I made a few nurses do a double take--by some standards I should have been started on Dopamine or had a fluid challenge but it all depends on the patient. If you have someone who is used to 300-400 blood sugars and you lower theirs to 80 I am willing to bet you are going to make them sick.
All of us are different. Always mention any concern to your doctor but if you have done this all of your life, have never had problems, and have gotten checkups I wouldnt be too worried. If there isnt a good reason for the tachycardia (like exercising

) then you may want to have that checked out.