My Garmin 500 consistently reports about 8% less than my riding buddy's 820. Both of ours show less climbing that what a route laid out in RidewithGPS will calculate in South Orange County, although other areas seem more accurate. For example I mapped out my GMR route from Saturday. RWGPS say 4,848', my Garmin reported 4,774, pretty close. I have RWGPS linked with Garmin Connect, so if i look at the actual ride, RWGPS says 4,714' After applying the RWGPS elevation correction, it says 4,831'.
Short of hiring a survey team, hard to know what the "right" number is.