The reason is pretty simple and something that will be likely overlooked by majority of new owners of the handset: there is a thin strip of transparent plastic covering the LED flash and the hear rate monitor.
The plastic strip has been installed a bit too well by Samsung, and most owners of the Galaxy S6 or Galaxy S6 edge will likely overlook that it is present in the first place.
There is a similar plastic covering installed on the camera lens, but there is a cutout in the center to make sure that the camera performance is not affected.
How are you liking your new Galaxy? Did you notice any improvement in the heart rate sensor’s performance after removing the plastic film?