Isogonic disagreement?

[Rolfrv]

On the Sectional and the WAC the Isogonic line near Petaluma (O69) shows 14*30" E. The airport info in the Garmin Pilot app, the Wing X Pro app, and the Variation at AirNav.com shows 16*E. AirNav shows it as 16*E(1985).
So why the variation? (pun intended)
Enquiring mind need to know!

Rolf
N9EY

 

[Jim Logajan]

Bob Gardner made a nicely informative post (thanks to an email he shared from a government cartographer) about isogonic drift and how often the lines on sectionals are updated in post 5 of the following thread (almost exactly one year ago):

http://www.pilotsofamerica.com/forum/showthread.php?t=47451