Hi Pato
In everyday aviation, declination and variation are used interchangeably (and notably with one term being more common than the other depending on which side of the Atlantic you find yourself on). Within the specific context of VOR's, deviation is the difference between system north and true north, variation is the difference between magnetic north at that geographic location and true north. The latter will change over time, the former will not unless manually adjusted at the station. AIUI.
Hi Scifly
Presumably then the VOR you refer to has never had an error so great that adjustment need be necessary. Bear in mind that an adjustment will have implications, in that IFR charts will need amending, any IAP's referencing that navaid will need amending, etc etc. It's relatively simple to adjust the station, it's all the stuff that need be done after (flight checking, amending AIP, docs, charts etc) which tends to result it in being done only when beyond a certain tolerance.
I have some source documentation for this but it's too large to upload so I did a Google search for some references instead. As it happens, the largest number of hits are from Pprune on occasions this has been asked before. More reading below for those who might wish follows:
As I said before, there are a few companies that do navaid calibration, but a few years back there was only really one in the UK, Flight Precision, callsign 'Calibrator'. I'm sure there are some ex-Calibrators kicking about these forums, perhaps they might chime in.
Cheers all
Charley