I ride with a Garmin 1000, and with Garmin Connect turned on.
Today I rode that segment at one minute forty seconds according to my Garmin
Strava at the end of my ride, it read one minute and 43 seconds.
Originally Posted by
DrIsotope
Oh, it's definitely Strava
So in one second, moving at say 20mph, your GPS inaccuracy could be 60+ feet. Strava will generally move you back to whatever the last GPS point was before you entered the segment, and do the same for the end
Actually, the issue is
GARMIN.
Here is a segment with the opposite issue.
https://www.strava.com/segments/12973075
https://www.strava.com/activities/258703543#16784982100
KOM: 34.2 MPH, 47s.
Max speed on segment: 34.0 MPH
And on the analysis page: Average: 32.6 MPH.
So, according to Strava, the average for the KOM is faster than the Max speed. The result is that it makes it real tough to catch the KOM.
In this case, the cyclist is using a Garmin Edge 200 that takes a reading about every 5 seconds.
Strava seems to choose the start/stop point to the nearest 5 seconds (no idea why it isn't divisible by 5).
So, if you're lucky, Strava knocks off about 2 seconds on each end, and one's time ends up about 4 seconds low.
If you're unlucky, Strava adds about 2 seconds on each end, and one ends up about 4 seconds high.
Ideally, Strava would interpolate speed/movement. Although that wouldn't be ideal as speed may not be constant. Standing starts?
TO FIX 
Some GARMIN devices apparently let you set the number of readings down to 1/second, or perhaps allows you to turn on or off "battery savings mode".