Polar vs Strava calories diffirence.

Hi i use polar grit 2 pro and recording my training session both by the polar flow and strawa at the same. Time (my i door cycling device is interactive and i sent HR from watch to it and the device is sending other larameters lien watts, distance etc. So basically strava gets all data from polar. But. After each session strava shows 50-60% what polar shows from the training. Today is 830kcal on polar vs. 450 on strava after 1h ride most of the time in 4th HR zone.

Anyone kniws which data is more reliable?