I decided to do a review of my last 5 rides or so using Strava at Beatty.  I park in the tennis court lot and ride the whole loop, including the new section and the black.  Takes me typically 43-47 mins for the whole deal at my typically cruiser pace.   That said, the mileage results from Strave are so bizarre.   4.9, 5.2, 5.9, and recently a 4.7.    I believe that with the new section it is 5.8 or close, can you guys confirm?  Does anyone else see similar results when riding the trail?  Starve ride info is posted belwo

GPS is inaccurate for mountain biking.  It's pretty much a fact of life.  The switch backs and elevation changes are too sharp for gps to catch everything.  It can be as much as 10% off, so 1 mile on a windy trail could be feasible.  My hypothesis is that it is particularly bad in the fall to winter time.  I don't have any evidence or explanation, but my experience is that I have my biggest gps anomolies this time of year.  It seems a bit counter intuitive since leaves being down should give you better line of site to the satellites.  Maybe the position of the satellites.  get a wheel sensor to improve the accuracy.

I've tried a couple of apps in the past with poor mileage results, but I'm currently using MapMyRide and it seems to be fairly accurate and consistent. I've been using it over a year now. 
I did three laps at Beatty a couple of weeks ago, including both black sections and averaged 5.93 miles.

+1 on the wheel sensor.  If you are trying to compare efforts or your efforts to someone else's, always look a the time not the average speed.  Time is constant for everybody.  If you want to track accurate mileage on the MTB a wheel sensor is a must.

6.7 Miles.....Period