Vivian Creek to San Gorgonio: What is the REAL RT distance?

There seems to be some discrepancy over how far it is from the Vivian Creek trailhead, up to the top of San Gorgonio and back again. Here are what various sites say the mileage is:

1. 17.3 miles - GPS data from my Garmin eTrex on this hike
2. 18.6 miles - The San Gorgonio Wilderness Association's official mileage.
3. 16 miles - LocalHikes.com

There may be others, but you can see, there's a pretty wide range of discrepancy -- nearly 3 miles! So what IS the actual distance? How do we settle this?