Our perception of physical exertion can be very subjective, based heavily upon our proficiency in a given exercise, as well as other factors such as fatigue, mood, and anxiety. If we are regular runners but only occasional hikers, for example, our threshold training runs may feel quite comfortable compared with hikes that, despite not elevating our heart rate so greatly, are in every other way more strenuous and demanding. We can generally push our heart rates to much higher levels in disciplines in which we are highly trained.
That is one possible explanation, but that said, heart-rate monitors certainly can be unreliable at times. Anything that I might say here is going to be conjecture.
I would recommend estimating your heart rate manually for each, then comparing notes. The radial pulse is usually easy to find with experience, but it can be weak in certain individuals, and take some practice to find. (If you are in the latter camp, it is not recommended that you use the carotid pulse, as this can lead to serious complications.)
I hope that is helpful.