I feel my calorie burned are wrong and very high for the activity ?

I do a walk of around 3.5 kms at a heart rate of 100-130.

At the end of the walk my stats tell me I lost around 500 calories. Which seems to me bit too many for a 40 mins walk?

How do I make sure that it is not calculating it wrong ?

I have Garmin Fenix.

Thanks