I'm having this issue currently. SInce this post is eight years old, I'm thinking a fix is not to be expected any time soon.
You wrote in the other thread, that your troubles with the Calories only started in January, so it does not look like it could be this 8 years old issue.
Garmin has a complete mess on this on all their gadgets. I have a garmin edge 1030 and a 935 I have had similar problems with, but Garmin is messing things up also. Their calculations is shifting from time to time. So I simply gave up looking at the numbers and make my own estimations.
When you cycle and you use the power pedals you also get a completely wrong value. It is only calculating the joules it takes to pedal the distance. Not the energy it takes to keep your body alive and then exerting power to the pedals. In reality you use more energy to do the job than just the walk.
It is not like you exert 1 joule on the pedals and consume 1 joule in food.
The relationship is more like 3-4x the food energy to generate the energy created on the pedal to generate the energy to create the energy on the pedal.
The same is valid for moving your feet and walking.
It is like a lamp. It takes more energy to generate light and overcome the resistance in the cord. If you get 60w on a lamp you likely put 2-3x the energy to make the lamp shine.
This means there is not a perfect conversion of energy to light for the lamp and any other process. You need to consume some energy to create the energy due to an imperfect world. The same is valid for food and body energy converted to mechanical energy, be it walking or pedaling energy.
This however is very individual. It depends on your body mass and your ability to create and absorb energy. All is of course similar between all indivuiiduals and should not vary that much.
You are however likely to produce a lot more than 350 calories to walk 10km
you can get more about this by reading on the internet about this but you also get a feeling of the levels if you read the ingredients and their energy level on a bottle/package of milk, jam, bread or any other food.
Then you quickly gain an insight that if you used 350 calories only you would grow overweight rapidly.
Unit conversion is one of the challenges here and then understanding how much your body consumes to generate energy. To move your legs and walk is the next level of challenge to understand - how much energy do you consume to create the walk, a lot more than 350 cal
it is not all correct as it depends on uphill, downhill, rain, dry, heat, cold etc, but gives you a feeling of the amplitude of your energy consumption to do the walk itself.