That's a typical range. If I use three different GPS activities apps I get four different readings. But I'd consider the results you got to be within an acceptable range.

GPS is remarkably accurate but cannot be as precise as mechanical measurement. There are variations due to GPS sync and other factors.

When in doubt, add another GPS activity app. Try Wahoo Fitness, a good free app that can upload to Strava. Running Wahoo Fitness and Strava simultaneously you'll get slightly differing results. Not worth worrying about.

Also, the on-bike computer will continue recording as you wheel the bike indoors. There's no auto-pause/resume. It's physical motion. It can be "paused" only by lifting the wheel or moving the sensor away from the magnet.

Strava, Wahoo Fitness and other apps use different thresholds to trigger auto-pause/resume. If I walk quickly enough indoors while wheeling the bike, it resumes recording and factors a slow speed into my overall average.

Usually after a ride I crop out approximately 400-800 yards from the beginning and end of a ride for Strava. That's because I'm often meandering around getting warmed up, stopping to chat with neighbors, etc. Sometimes I coast around in circles while chattering with neighbors. So those bits aren't useful workout info.