You do not say how much it varies. Is it a few beats per minute or 20 BPM? Are you just sitting and relaxing the whole time you take your BP/HR and nothing else changes yet you see your HR move from say 70 to 90 while you are just sitting doing nothing? Or are there other factors including if you have been physically working hard before hand, time of day or night, have you just eaten or woken from a poor nights rest?