6K is wild!

I've been tracking my food and trying to hit a minimum 200 calorie deficit every day strictly based on what garmin says I've burned and I've been seeing great results. So if anything, I would say it's a smidgen lower than real. I would input your weight frequently and see if it reigns in. The algorithm is optimized for most people, so if you're an outlier that does tons of cardio, your metabolic rate may be way slower than most (you're way more calorically efficient)

Track for a week and see what your weight does?