The heartrates I achieve in the summer are higher than I see in the winter. Heat and dehydration are factoring in.

I also get a higher heartrate while cycling than I have when swimming. The cool water and horizontal position make it easier on the heart.
Swimming butterfly I still don't get the same heartrate as cycling uphill. (I have a competitive swimming background)

I will hit higher heartrates on a ride if I have had a fear episode earlier that day - driving/riding in scary traffic or being chased by a dog.
A hard hill that I struggle on also makes me more aggressive and I cannot ride socially afterwards, I just want to hammer not yammer.

I took a maximal stress test age 37 and have hit the same heartrates on some steep hills this summer age 57.
When I was a teenager I would hit much higher heartrates so there has been some decline over the years.