Calories burned - Strava V Heart Rate Monitor

Hi folks, i am looking for some advice. when i'm doing exercise, i run strava on my phone for distance/speed etc and a heart rate monitor to monitor my heart and calories.

however, strava also records calories, and always is always different by over 200 calories than the HRM. but the weird thing is, sometimes it is less on the strava and sometimes more.

which one is more accurate? both are set up to my weight, age etc, so there shouldnt be such a disparity.

thanks in advance for any advice :-D