Difference in active calories heart rate strap vs. only watch

Hi all,

I recently purchased a heart rate strap in order to have more accurate HR reading whilst cycling indoors. I have noticed that my average HR for a similar activity is very close in both cases (with watch + HR Strap / WIthout HR Strap and only watch). However I have a large difference (2x more for HR Strap) in my Active Calores (not activity calories though..)

Does anyone know if the HR strap has a different way of calculating active calories than the watch?

Thanks!