In reply to elsewhere:
Are start and end points identical?
Do you both have Strava set to snap to ground (using a DEM)?
Are they mounted similarly?
Random errors in position fix all add up. The noisier the signal, the greater the random walk effect. Noise is affected by the mounting; the ability of the antenna to get a good, clear view of the entire sky, horizon to horizon. Covering the antenna, especially with wet fabric, will attenuate the signal, increasing position noise.
Are they both recording at the same rate (consumer GNSS gives 1 position per second, but logs can be set slower than that, to save memory). A lower recording rate will 'cut corners', and reduce the effect of random walk.
Elevation is the least accurate of GNSS measurements, due to the geometry of the constellation and receiver.