Oh, it's definitely Strava. They "reinterpret" your GPS data during upload. Every segment has a defined GPS coordinate for it's start and finish. Unfortunately for us, the precision of consumer GPS units is about 10 meters, and can be worse at speed. 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-- pick the closest coordinate after the end. So you can cover an additional 60+ feet and get no credit.

Sometimes it falls in your favor, mostly it doesn't. Strava adds time to my segments and rides pretty much daily. I've KOM'd a segment by two seconds (via Garmin) and after upload got a tie for first. I recently lost a KOM where I in the compare efforts graphic, I was ahead by 5+ seconds with a few hundred yards to go, then the new 1st place just shot forward like he could ignore physics-- speed didn't change, he just covered the distance instantly. GPS errored in his favor that day.