Which activity are you better at cycling or running? 80% of your maximum heart rate may be at a lower exercise intensity from one activity to another.
It is possible that you would burn more calories from one activity to another because you are better at it and can work at a higher intensity.
I so wish for you it was cut and dry, that 80% of your maximum heart rate burns X amount of calories whatever the activity, but that is not the case. You got to take skill level among other things into consideration.