How can the track TIME be different on the device than it is once sync'd to Explore.garmin.com?

Explore.Garmin.com always shows a different (longer) track time on my tracks than shows on the device. I understand that there can be variance in track DISTANCE because of logging timing, etc. but how can track TIME possibly be different?  I START the track timing and END the tracking on the device. How can there be a variance in Track time once up on Explore.garmin?