Apple Watch fitness calories not accurate

Hello i recently purchased a s6 watch its been working perfectly, so my girlfriend got persuaded into buying an SE version mainly for her fitness needs. We recently started working out together and her watch shows that she only burns half the calories that i burn. So we swapped watches, she had mine and i had hers for todays workout and the same thing happened. She burned twice as much calories but with my watch, could this be because the sensors underneath are different for the s6 and SE model? Thanks in advance hope someone can help.