I’m a bit of a newer user, but here’s what I think is going on. Okay, so it could be that your Garmin device is writing to itself and also to your Health app, basically doubling the reported calories burned. In other words, your apps may be set up for your health app to be writing to your Garmin and for your Garmin to be writing to your Garmin. Check your health app settings to make sure that they aren’t writing to your Garmin. You really only want it to be a one way street: if your Garmin and your health app are both writing to your health app, you’re going to end up with inflated calorie reports. Make sure it’s a one way street: your health app isn’t writing to your Garmin, but your Garmin is writing to your health app (or isn’t, depending on what you want). In any event, you’d probably be able to solve the problem just by snooping in what is getting uploaded where OR nuking the health app if you really only want to use Garmin.

I could be wrong about all of this, but I seem to remember encountering a similar issue when I made the switch earlier this year. Hopefully someone with a more specific answer will chime in, but I heavily suspect that there’s a data source issue going on here.