Open-water swim distance differs from the track recorded? (Todays swimrun 352m vs. approx 750m, unacceptable)

I just don't get these discrepancies. There can only be two things going wrong. There's a sensor fault or your stroke style doesn't fit in with Garmin's idea of freestyle swimming (unlikely). I did a half ironman today and my swim distance was correct to one meter. Am I just one of the lucky ones that got a functional watch?

connect.garmin.com/.../9083518028

I don't pretty much believe either of your explanations. With the swimrun I was using hand paddles, it's kind of normal. That might mess things up, but there should not be some stupid algorithm trying to figure out distance from strokes/stroke styles as the GPS has pretty good data on the distance.

I think Garmin has some weird idea if they are trying to "fix" openwater distance with some swimming stroke algorithm and should at least allow turning that stupid thing off if one exists. That seems to be only logical explanation I've come up. Is that then because of sensor fault or stroke style is irrelevant, there should not be a thing at the first place.

The GPS tracks have the correct distance, so clearly neither of those is applicable. For whatever reason, Garmin's software messes up the distance from the track.

Measuring distance from strokes makes no sense, that's wildly different from person to person as everyone who has ever been near a freestyle swimming clinic can tell you.

Looked another swim.. Garmin distance 250m.
https://connect.garmin.com/modern/activity/9073240184/24 

Doesn't look that the GPS lock was staying, but as it's a swim where I'm going in and going out from the water when it at least gets GPS lock, not doing some "oh, my estimation is 250m, but the distance from the swimming start point from to the endpoint is 500m, my estimation can't be any good, maybe I should use the distance from starting point to end point, not something half the distance".

It seems the difference has to do with file type.

- fit file = completely off

- gpx file = spot on

What tool(s) are you using to look fit or gpx? If I look my .fit with GPXSee I can see distances as they should be, but it might be that the tool does something and calculates the total distance and then Garmin is cutting some distances away if it has moved a lot and not counting that movement as distance or something like that? 

I use Golden Cheetah software:

https://www.goldencheetah.org/

I export the activity from garmin connect in fit and gpx format.

Then i import the two formats in Golden Cheetah.

This data makes me second guess the stroke data being the thing that causes the much bigger failure in swimrun. Hard to explain with 3 stokes/min difference these numbers...

Yesterday swimrun and couple of swims there:
Garmin 280m, GPX 507m. Swimming pace 4:34/100m, 18 strokes/min
Garmin 400m, GPX 745m, Swimming pace 4:44/100m, 18 strokes/min

Today, plain openwater:
Garmin 4248m, GPX 4350m, Swimming pace 2:43/100m, 21 strokes/min

And on a sidenote, plain openwater swim was pretty decent, just about 100m difference, but the swimruns are total mess.

same for me, Garmin shows 736 m of my yesterday open water swim, where the distance measured on Google Maps, along the Garmin's track, shows 1200 m. Track is displayed correct, but the distance way under-reported.

Garmin:

Google Maps:

I know it doesn't help those who are having issues, but I've been very impressed with the accuracy of the OWS (Multi-band). I've done several swims and the recorded distances are very close to the real distance. This is all frontcrawl with firmware 10.10.

The lap of our local lake is ~550m, with a shorter, inner lap of ~250m.

In this example I did 5 outer laps and 1 inner (3000m). Garmin recorded 3030m. In another swim it recorded 2978m