Heart rate data differences

I have a Foreruner 935. I've noticed that the heart rate data looks very different depending on whether I am using the optical heart rate monitor or the chest strap monitor. I've attached two pictures. The data looks very boxy when I wear the strap, and more spiked when I just use the watch's optical heart rate monitor. Is there a reason for this? Which one is showing a more accurate reflection of my heart rate? Thanks for the help. ciq.forums.garmin.com/.../1369799.png ciq.forums.garmin.com/.../1369800.png