poor GPS accuracy?

You need to be a little careful in what you are viewing the activity with to make GPS accuracy claims. Garmin Connect Web will down grade the data when drawing the plots so as not to pull thousands of data points. This results in corners being cut etc.

5Km over 300Km is not that much, 1.66%.

Did either of the units have a speed sensor?

If they are not calibrated then the distance between units will vary.

The track points from both devices are in different places on the same road, so it’s not a GPS accuracy issue. The straight lines are simply joining the points together and when moving fast the distance traveled between them is greater than when moving slower.

ahh , that will be culprit. both are on "auto" or "manual" or different setting in the wheel circumference submenu? the standard 2096 or 2105 numbers usually not accurate (the exact number is depending even the tyre wear, between a brand new tyre vs used one 1% or more is easily exists, so 2 same bikes with same 25mm tyre models but with different tyre wear gives you totally different distances after 300km with same manual "2105" value)

alright, seems logic :) however ... shouldn't distance be checked and corrected in Connect/Strava etc? I can set wheel circumference extremly large, ride 1 minute and took ... 50km? Programs like Connect or Strava should correct distance based on maps

It's not the GPS data that's inaccurate.  It's the tool/site you're using to view it.  The site skips many points in plotting the line.  I've seen this myself depending on the site I'm using. Try a different site.

the strava is correcting it only if you press the "distance correction", it trusts in the FIT file distance data (which is coming in this case from the speed sensor, if it is not present, then from the GPS distance).  yes, you can ride with miscalibrated speed sensor so fast on Strava :)