For Info in the "Real World" when a VOR is first installed, the VOR antenna is physically oriented to True North, this involved a TELS enginer climbing to the top of the mountain (or where ever the VOR was installed), with a set of screwdrivers and spanners/wrenches, then making a potentiometer adjustment to slave the navaid with Magnetic North. This setting is then recorded as "The Magnetic Declination of Record" or as it is sometimes called the “Station Declination” and the date this figure was set.
Now over time the Local Magnetic Variation changes, and the VOR will no longer be alligned exactly with Local Magnetic North.
Every so often the controling local Aviation Authority should re-align the VOR antennas to keep then within a few degrees of Local Magnetic North. But given the remote locations, and the fact that all charts & FMC databases would have to be updated as well, lots of VOR's do not get re-aligned as often as the should.
The FAA and other Control Authorities continually have problems with "MagVar of Record" for VOR's and other navaids in the "real world".
As example: Instrument Landing System (ILS) procedures at Anchorage and Fairbanks, Alaska. and The Review of Magnetic Variation Report.
So it was a never ending job, trying to travel to every single VOR ground transmit antenna and "tweek the pot", given that some VOR's are in very remote or mountainous locations. The FAA has tried to keep most of it's VOR's in line, but there are many other VOR's around the world that have hardly ever been re-set more than a couple of times, so now lots of these VOR's are way out. This is one of the reasons Control Authorities are planning of switching off most ground based en-route navigation aids VOR/NDB/LM ect. and replace their function with satellite based fixes.
Also Have a look at these old posts on the X-Plane Dev Blog: http://developer.x-plane.com/2015/06/x-plane-10-40-beta-3-is-out/#comment-11146
Note especially what Phillip has to say on the subject:
X-Plane NavData records this "Site Declination of Record" in the earth_nav.dat file, Row 3 entry for VOR's as "Slaved variation for VOR".
cessna729.