Where are you getting the calories that you are assuming are correct? (Twice what the watch is telling you). I have my A2 connected to a heart rate monitor strap. My HRM tells me 600 calories (assuming this is correct since it's from a hrm). Samsung health however tells me I burned 1200 calories, which I know is incorrect. Samsung health seems to be doubling calories burned. No idea why. That's why I asked. If you're thinking you are burning twice the calories, I hope you're not getting the higher number from samsung health.
Average bpm is way too little info for anything to give you an accurate estimation of calorie burn. Unless you are entering way more info than just bpm, your watch is likely closer to accurate than an online calculator. If you want accuracy, get an external HRM.