Your watch says the session burned 600 calories. It probably did not. Wrist-worn devices are decent at counting steps and reasonable at heart rate, and their calorie figures are the least reliable thing on the screen.
What the device is actually doing
No tracker measures calories. It cannot. Measuring energy expenditure properly means capturing the air you breathe out and analysing its oxygen content, which requires a mask and a lab.
What the device does instead is estimate. It takes your age, sex, height and weight, adds movement data from the accelerometer and heart rate from the optical sensor, and runs the lot through a model built on population averages.
Every step of that chain introduces error, and the errors do not cancel out neatly.
Why the estimates lean high
Population averages are the first problem. The model assumes you burn like a typical person of your stats, and your basal metabolic rate can sit meaningfully above or below that average for reasons no wristband can detect.
Body composition is the second. Two people of identical height, weight, age and sex can carry very different amounts of muscle, which changes their resting burn. The tracker cannot see inside you.
Then there is the double counting problem, and this one catches almost everybody. Your device reports the calories burned during a workout. But you were going to burn calories during that hour regardless, just by being alive. Unless the number is adjusted for what you would have spent anyway, part of your reported burn is energy you were spending on the sofa.
Where the sensors struggle
Optical heart rate at the wrist works by shining light through skin and reading the reflection. Movement disturbs it, and it lags behind rapid changes in effort. Intervals are exactly the case it handles worst, and cold hands make it worse still.
Accelerometers assume arm movement means body movement. Push a shopping trolley or hold a handrail on a treadmill and your effort disappears from the record. Wave your arms while sitting and you get credit for steps you never took.
Some activities defeat the whole approach. Weight training raises heart rate through effort and strain rather than sustained aerobic work, and heart rate based models tend to read that as more energy than it costs. Cycling, rowing and anything with static loading all have their own problems.
How wrong is wrong
The gap varies by device and activity, and it is often large enough to matter. Errors in the tens of percent are common in the research, and the direction is usually the same one. Heart rate itself is measured fairly well on most modern devices. It is the conversion from heart rate to calories where the accuracy falls away. [ADD SPECIFIC: a validation study you can cite, with the device and the error range]
Why it matters more than it sounds
Because people eat the number back.
A 600 calorie estimate for a session that actually cost 350 gives you 250 phantom calories, and if you use them as licence to eat, you have quietly wiped out a moderate daily deficit. Do that four times a week and a diet that should be working stops working, with no obvious explanation.
There is a motivational cost too. The inflated figure makes progress feel like it should be faster than it is, and when the scale disagrees, people conclude their metabolism is broken rather than that their watch is optimistic.
How to use a tracker sensibly
Treat the calorie number as a relative measure, not an absolute one. It is fine for comparing this week's training load to last week's. It is not fine as an input to your food.
Set your intake from your maintenance calories and let the scale correct you over a few weeks. That feedback loop is slower than a live readout and enormously more honest.
Do not eat back exercise calories, or eat back at most half of them if you must.
Enter your details properly, including current weight, since a profile you set up two years ago is working from a body that no longer exists.
And keep the parts these devices are genuinely good at: step counts, heart rate trends, sleep patterns, and the daily prompt that gets you moving. Those are worth the money. The calorie figure is the one number on the screen you should argue with.