I've had a few recent inquiries from athletes as to why their heart rates are lower in the aero position on the bike as opposed to sitting up. It's all about gravity. When in a lower position, there's less gravitational pull on the blood returning to the heart. Increased venous return, aka amount of blood returning to the heart from the periphery, equals less pumping of the heart (lower heart rate) to get the blood it needs. This is the same reason why after a hard session, you see runners put their hands on their knees. Decreased gravity, increased blood flow back to the heart.