It's probably been that way your entire life and your just finding out about it because you bought a HRM and you're looking at the data? probably has something to do with endocrine and sinus arrhythmia but these are completely normal. I've monitored my pulse for training and I had to stop because I was overthinking it and making problems for myself where none existed.
Just my humble opinion, and the opinion of my friends who also run marathons, monitoring pulse for training purposes has very limited value. Two of my friends just ran Boston, 2:50:00 runners (6:30 average pace). Neither monitors pulse for training, they monitor pace. They've monitored pulse in the past but it turned out to be of little value and amounted to a curiosity.
Monitoring pace tells the bigger picture.
And it's not just your heart moving blood during this type of activity, there's also "muscle milking" a.k.a. skeletal-muscle pump activity. I think this is why some people say they benefit from compression garments, I could see how it would assist with this but I'm guessing.