difference between Locus and Garmin track length

[Will C]

Morning, I have a question regarding the difference I have noticed in track length recording between my garmin forerunner 255 and my Locus map app. I did a walk yesterday and ran both at the same time. I walked either 9.7km or 10.9km (locus vs garmin). There are differences in length, elevation gained (locus vs garmin) 433 vs 509 but maximum elevation (locus vs garmin) 257m vs 268m. I know the platforms are different but this is over 10% difference, it's quite surprising to me. What can I do to find out which is most accurate?

Both tracks shown below.

[freischneider]

At the times when I still used Garmin, Garmin was very inaccurate. Only a few points were recorded and abbreviated in curves. When recording Locus you can set how often a point should be recorded.
For altitude, it is best to use the "1" altitude data from Sonny. A barometer would also be quite good.
What does your Garmin use for altitude data ? If it uses barometer, how often is this calibrated.