Cardiac output = HR x Stroke Volume.
If your left ventricle outputs less blood per beat than another person, then your heart rate will be higher (this assumes you have the same demand for oxygen for your muscles that the person you are comparing yourself to has). There is also the phenomena of heart rate drift which occurs when you become dehydrated over the course of a long endurance event. Less blood volume means that you will have less blood pumped out per beat (stroke volume) and thus your heart rate will drift upwards.
One test a physician can order is an echocardiogram which will evaluate the ejection fraction (what % of blood is pumped out of the ventricle with each beat) as well as the functioning of your heart valves. It also evaluates the thickness of the walls of the heart chambers. It is a non-invasive test.
ETA: I don't find it particularly useful to try to compare heart rates among individuals. But it is useful to keep track of your particular heart rate at a given effort and investigate if it is trending much higher than normal. Could be indicative of a structural issue or an electrical issue (a-fib comes to mind). Certainly worthwhile discussing with your physician.
Last edited by GadgetGirlIL; 06-17-19 at 11:57 AM.