High heart rate = inaccurate calories burned?

[Boo12]

I've been using my Fitbit blaze for almost a week now, mainly for the purpose of tracking my steps and long walks that I go on most days. I am trying to lose weight so I have also synced it with MyFitnessPal. 

My resting heart rate is around 65bpm, but I am noticing that my heart rate increases dramatically with any kind of activity, which is causing the Fitbit to think I'm in 'fat burning' mode, even if I'm just sitting down doing my makeup. When I actually go for a proper walk, my heart rate is registering at an average rate of 135bpm, even if I'm not out of breath at all

I have increased my max heart rate settings on the app so that the heart rate zones are higher, but I'm still being told that I'm burning a ridiculous amount of calories each day. I.e. On a 5.8 mile walk I supposedly burned 1,389 calories, whereas if I put that into MyFitnessPal it suggests that it's closer to 800 calories. That's a huge discrepancy! 

I just want my calories to be as accurate as possible, so that I'm not unknowingly overeating. Does this all sound normal or is something wrong? 

[Goosse1962]

I don't know about the calories. However, I have seen what you appear to be saying about the exercise zones.

When I first started tracking, as well as twice when I was unable to exercise for a few weeks, I was very quickly in the cardio zone with very little movement. I easily hit peak when going for a walk. I was in fat burn almost all the time including during part of my sleep. Going for a walk 4 to 6 days a week, I started spending less time in the higher zones for the same activity level. I wasn't out of breath, just not having a good heart rate.

The higher my resting heart rate, the easier it is to get to the higher zones. The lower my resting heart rate, the more effort I need to put into exercise to get my 30 minutes of cardio and few minutes of peak.

[AlejandraFitbit]

It's great to see new faces around @Boo12 welcome and @Goosse1962 thanks for stopping by.

If you are having issues with your heart rate reading, I recommend checking this post, where you will be able to see some things that might be affecting this reading and some tips to improve it.

I hope this helps, let me know the outcome.