Garmin Connect with Apple Fitness app

It is not only problem with this app.
Yesterday I made 13000 steps but app says that I did 24km. Today it looks better but for some day last week I also saw such big discrepancy.

This is the wrong forum, it would go better in the Garmin Connect Mobile for iPhone forum.

i have the same issue. I think it is a problem with Garmin Connect. If i check the data in Apple Health it shows for the workout not the actual time i did the workout, but the time the workout was synced from Garmin to Health. That way Health cannot deduplicate the workouts. So it sees one workout recorded from the phone and another workout synced in using Garmin Connect.

The problem is that Garmin Connect does not correctly implement most of the Apple HealthKit APIs.  Garmin has known about the problem for years, and a representative has commented in the forums that they have no plans at this time to fix any of the issues.  If you search the forums you will find other folks who are unhappy with this.   As a workaround you can get RunGap or HealthFit from the App Store to sync workout information, but there is no solution for most of the all day metrics. 

I know about the problem that Garmin (a multi billion company) is not willing to implement this correctly. 
But just accepting this situation is not ok. Their products are not cheap at all, the marketing mentions the feature, so we can expect that it works correctly.

It turns out that Apple Fitness counts steps from the iPhone and Garmin Connect simultaneously, which is why it gives inflated results. The following setting in Iphone helped:

Settings ->> Privacy ->> Movement & Fitness ->> Fitness Tracking OFF.

Now data from the iPhone does not count towards Apple Health, only data from Garmin Connect.