Disappointing GPS Accuracy

I just noticed on Strava that some collegues running the same halfmarathon have pretty better accuracy (even with some Garmins (forerunner 935, fenix 5, but no fenix 5splus).

Start and stop routine is easy: you cross the rfid bar and you start and stop the watch. I would assume there can be an errors less than 2m.
my pixel and fitbit ionic was pretty accurate. The meassurement was official because of champignonship

it is no problem. Next time i will use runtastic and my smartphone as well as my fitbit to control my running pace to hit some specified targets.

my pixel and fitbit ionic was pretty accurate

Define 'pretty accurate'. What were the distances measured? Your Garmin is by any account 'pretty accurate' for a consumer device.

@soss3sp as per the above post - the accuracy is well within bounds of what should be expected. On an officially measured course generally the distance that you run will be greater than the measured distance (unless you cheat :) ) . I have run the occasional course where the measured line is actually painted on the road and this is probably the only situation where it is possible to run the measured distance. You don't need to deviate too much from the line to add a few percent to the distance - also you talk alot about pace - do you mean instant pace or overall average pace. If the first then GPS is very poor for that and you should consider a foot pod - if the latter then it will be reasonably accurate - but again subject to the % error that you are seeing.

I think the distance reported by the F5+ is totally acceptable, because - as others have pointed out - 21.095 km will be the absolute shortest distance you can run without leaving the course and because random GPS error will usually cause a slight increase in distance. I'd be a lot more annoyed if I came in short an miss a Strava Badge for my effort. ;) If you want/need accurate instant pace get a Stryd footpod.

In addition have a look at the Peter's Race Pacer data field to keep you updated during the race with regards to the expected outcome and your goals

Thank you to all for your responses, i speak about overall average pace (per km) of course not about instant pace.
As I mentioned before, I will just use my other devices which have been closer to the halfmarathon distance. It is not a problem of the none ideal run.
When I run on official streets and the stadium I can easily see the deviations my Fenix does have while my Pixel is a lot more precise. The deviation of my garmin is huge enough to be unable to tell from the trace which side of a broad street you have been while running.

Thank you to all for your responses, i speak about overall average pace (per km) of course not about instant pace.
As I mentioned before, I will just use my other devices which have been closer to the halfmarathon distance. It is not a problem of the none ideal run.
When I run on official streets and the stadium I can easily see the deviations my Fenix does have while my Pixel is a lot more precise. The deviation of my garmin is huge enough to be unable to tell from the trace which side of a broad street you have been while running.

Do you assume that the phone is more accurate because the track looks better? Or just because you ran a "half marathon" and the number is closer to the official distance of a half marathon? I've never run a race where the official distance is spot on. It that would actually be very difficult to achieve. Might be your Garmin that is more accurate. Compare them on an actual map with satellite imagery and see. When you say you can't tell the side of the street, are you looking at a satellite image or street map?

My last 1/2 marathon worked out pretty well from a distance perspective.
https://connect.garmin.com/modern/activity/2678353198

I haven't run a 1/2 marathon race recently and probably won't soon, but I have nothing to complain about based on the distance recorded on my last effort.