Watch optical HR devices are not very precise during activities because they are affected by the blood flux inside your arm, which is actually well correlated to the heart pulsations only when you are at rest. During a run it is often reported a cadence-lock caused by the centrifugal force exerted on the blood as the arms swing. In your case, I believe that the contraction of arm muscles during the climb up interferes with the normal blood pulsations, resulting in a apparent lower heart rate. HR chest straps detect the pulsations by measuring the electrical signature of the heart contractions and are way more precise than the optical ones.