Magnetic Declination can be the cause of these errors, as rightly suggested.
When a VOR is installed or receives a regular calibration, it will have its datum realigned with magnetic north. Afterwards the VOR will be flight tested by one of the navaid calibration companies; Flight Precision used to do it with King Airs but I gather there's a few companies now, e.g. Cobham.
The information you 'need' (if you want to be geeky about it) is all in the AIP.
For instance, this is the entry for Belfast VOR:
BELFAST
VOR/DME
(2.9°W (2019))
(decl.: 4.0°W)
The magnetic variation is suggested as 2.9W for 2019 but the declination is still set to 4W. Therefore you can expect to see a 1° error between mag bearings and radials for the time being, until it is datum is next realigned.
So, strictly speaking, radials are not referenced to either true north or magnetic north; radials are referenced only to the datum at the VOR. Kudos to eckhard for bringing it up.