Difference between Polar Flow app data and Apple Health data

hi, I have a Polar m430 watch and an Iphone 8. It occures many times, that I use my watch all the day and even on the following day (just to make sure everything is synchronised) resting energy + active energy in apple health differs from polar flow app calories data. I have checked the source of the data, all the energy data came from polar flow to health. You have any idea how could it happen? It really bothers me and no forum answered this problem.