Variability in heart rate during exertion, digestion and other processes is an entirely normal physiological response and despite what individuals may sense as inconsistency, the particular combination of influences upon the body at any given time virtually always produces necessary changes in order to maintain homeostasis, or normality. All manner of factors all play a role in how the heart responds to these influences and it is not an indication of a problem at all.

It's important to realize that the heart's ability to compensate under variable conditions is a sign of a normal, healthy heart and the extent of compensation is not gauged by whether you perceive it to be commensurate or appropriate, but rather by the status of other physiological functions taking place at the time.

Nothing wrong here at all.

Best regards and Good Health