Fitness app and workout app logging different amounts of calories during workout

When I am at the gym I use a fitness app called Fitbod. My Apple Watch automatically starts tracking my workout as well. When complete, the Apple Watch calories burned and the fitness app calories burned are WAY different. Apple Watch is much lower. Like today I worked out for over an hour doing a circuit/functional fitness routine. My fitness app says my workout burned over 635 calories. Apple Watch says 159. That’s crazy different. Which one is accurate?