WHOOP relies heavily on continuous heart rate monitoring for calorie burn estimates. Higher HR generally correlates with increased calorie burn, but WHOOP will adjust to you over time. If you notice a major change in your calorie burn count, ensure WHOOP is flat on your arm, minimizing movement even during activity for accurate HR tracking.
Additionally, WHOOP also uses factors like age, sex, weight, and height - which all affect basal metabolic rate (BMR, the calories you burn at rest) and energy expenditure and are included in the algorithm.
One thing to keep in mind with calorie burn numbers from ANY wearable device, including WHOOP, is that it is intended as a relative estimate. While WHOOP’s calorie tracking is highly accurate, it is recommended that you utilize the calorie burn data as an estimate rather than a true clinical measure. I would not base fueling entirely on a single calorie burn number, but it can be useful to know days where you are relatively more or less active and may want to consume more/less fuel.