canklecat

It's mainly how the apps/sites interpret the GPS data. I've simultaneously run Strava, Cyclemeter and Wahoo Fitness on the same phone (months ago when all three would run together -- that hasn't worked reliably the past couple of months).

Each shows slightly different speeds and distances, and the map route traces each look slightly different. And when there's a GPS sync error, each app differs in guesstimates of my likeliest route, speed and distance.

Incidentally, GPS sync errors may be the single most common challenge to getting consistent data from any GPS device and app that interprets the device's data. GPS errors appear to occur more often than we realize, although the apps tend to smooth out those errors with guesstimates about the most likely data between recorded GPS data before and after sync errors occurred.

Generally, Cyclemeter showed me faster and riding a bit farther than Strava. But when the Cyclemeter data was transferred to Strava, Strava then interpolated the data differently, usually showing my average speed slower and distance shorter.

Strava explains this in on their website. I don't have the link at the moment but it's readily available via Google. Cyclemeter claims it's more accurate than Strava. But there's no way to determine any of this without a calibrated speedometer/odometer and accurate maps, including calculating your own grades/elevations if you don't trust the existing data. I don't see any reason to distrust the existing data for most areas, though. I've manually estimated grades on some routes I ride often, using iPhone apps to calculate the angle at various places on climbs (two of my bikes have horizontal top tubes, close enough for this purpose). My measurements matched the available maps and Strava data.