I don't think it is clear WHEN you are taking your heart rate.
For instance, if you are talking about your resting heart rate - you take that before you move out of bed. That should be considerably lower than after you've moved around even a little.
You should get close to your resting heart rate by just sitting still for, oh, say, 10 minutes, I think.
Something else that can impact this is your effort the day before. AFTER you have made a significant effort, your heart rate will stay elevated to a degree. And, if you are still feeling the effects of the previous day's workout, it should show in your heart rate (hr) and blood pressure (BP). If you have not yet recovered, your bp and hr would still be elevated somewhat. How much depends on how hard the workout was, and how much you have recovered.
It is of great use to the sailor to know the length of his line, though he cannot with it fathom all the depths of the ocean. ~ John Locke