Data consistency between Apple Health and Garmin

If Apple Health receiving correct data from Garmin Connect I would say, problem is on Apple Health side and suggest address it to Apple support.

Thank you for getting back to me !

I’ve already reached out to Apple Support, but I was wondering if this could be related to the way the data is being interpreted, since I’ve never seen this happen before. I thought maybe someone else might have experienced the same issue.

I think I know where is the source of your problem. 
In Garmin distance is taken from GPS readings during activity, in Apple Health distance is estimated from number of steps multiply by average step length. 
I suggest what is step length set in your Apple Health and I think mystery will be solved. 

I think the issue might be related to how Apple counts steps. Each sample in the data has a startDate and an endDate, and some of the samples coming from Garmin appear to overlap in time with samples recorded by the iPhone. It’s possible that, in these overlapping intervals, Apple prioritizes its own step data (from the iPhone) and partially or fully ignores the Garmin samples.

This is just a hypothesis, but it might be worth mentioning if you’ve already opened a ticket or are in contact with their support team.

Adam-1972 said:

If Apple Health receiving correct data from Garmin Connect I would say, problem is on Apple Health side and suggest address it to Apple support.

I think it really comes down to bad HealthKit implementation on Garmin's part. Apple Health just displays what it is given. It does not re-process like Strava likes to do.

Keep in mind that Apple will also use your phone to determine steps takes. It helps is you make sure you have Garmin as the primary source of data but it will still be wrong, at the end of the day, when compared to what is available on Garmin.

I also think this is more on the side of how and what data is Garmin writing to Apple Health.
I can see that Heart Rate samples in Apple Health are present once every 2 minutes or so, but Garmin has data for every second most likely. Not to mention the Garmin does not write Running Power or Running Speed to Apple Health... So much data is missing because it seems they do not want to write them. They do not even request access to write it.
Regarding steps, they write steps once every few minutes, maybe 10-15 min. These samples has a start and an end date which overlap with samples saved from iPhone and Apple most likely prioritize those samples in the same period of time.

I think Strava gets more data from Garmin than Apple does. Why?
I see Strava displays the correct average heart rate, for example, while Apple Health does not. And I checked the samples in Apple Health and computed the average on them. I get the same result as Apple does, caused by the small number of samples.

I think you are wrong. 
First, it is not Garmin writing data to Apple, it is opposite, Apple Health is pulling data from Garmin. 
If data are correct it indicates Apple Health is interpreting it wrong, for example is not pulling information from GPS about activity distance, just estimating distance based on number of steps and average step length. When you run, your average step length depends from your pace and clearly is longer then your step when you walk. 

As I mentioned in previous post. 
Garmin is writing zero data to Apple Health, it is Apple Health pulling data from Garmin. 
Blame Apple what data they get and how interpret them. 

Are you sure? Garmin Connect requested WRITE access to Apple Health, not the other way around.
Check this: https://support.garmin.com/en-US/?faq=lK5FPB9iPF5PXFkIpFlFPA 

You are right, my mistake.
I was convince that it work other way especially that Apple Health shows Garmin Connect as data source what I read that Apple is pulling data from Garmin not Garmin pushing data to Apple.