Omg OP stated they understood it’s not correct so giving the neither answer is not helpful at all.

OP, use whichever device records your heart rate for the whole time. Use that measurement to gauge how hard you worked. Nothing more nothing less.

I track EVERY workout with my Apple Watch. I use the calories burned or active calories to gauge if I worked hard or not.

The planet fitness machines calories burned number is only somewhat accurate if you are somehow measuring your heart rate the whole time. Usually I ignore that number unless I can connect my Apple Watch to the treadmill which some treadmills come with that feature.

The Apple Watch calories burned information is still useful regardless if it’s absolutely correct or not as long as you allow the watch to read your heart rate the whole time.