TDEE too low?

Hello there,

I recently started using a Forerunner 255, after being a loyal Fitbit customer for 10 years. To my surprise, my Forerunner gives me a total daily burned calorie count that is a lot lower than my Fitbit used to, to almost a 800-1000 cal difference.

To paint a picture: I am a CrossFit coach, coaching actively 4 hours of classes daily, biking to and from work (15 min each) , a daily training session of 1 hour and about 12.000 steps easily. 
I weigh 60kg at 1.74m. I move around a lot.

Yesterday it gave me a total of 1700 cal burned, which is actually my average BMR (calculated after a Dexa scan)
So how come the number is so low?