There are many reasons why, they are different, so many in fact you could even ask the question “should they ever match?
The two biggest factors are the autonomic nervous system, and aerobic off loading.
Aerobic off loading is the most likely reason. We know that we can create the same power for less physical force by increasing cadence. but we also know increasing cadence increases aerobic load and therefore heart rate. Try riding at a steady zone 2.5 power with a slow cadence for 5 min and watch tge heart rate. Rest 5 and try again for five with a cadence increase of 20 or 25. you should see a significant increase of heart rate for the same net power.
Whilst interesting you are less likely to see the affects of the autonomic nervous system switching from sympathetic to parasympathetic during a work out. You will in a well trained athlete doing extended /ultra endurance efforts. what happens is the body starts in flight ir fight mode and then switch back to a more sustainable mode
regards
J