There are two types of VOR, conventional and Doppler, they work differently. In CVOR the variable signal is directional (4 antennas or slots used as a directional array), thus for the receiver the amplitude of the signal changes, and is detected as AM by the receiver.

In DVOR the variable signal is transmitted on the 10k subcarrier, itself transmitted by two antennas part of the circular array. These two antennas are not directional, there is no need to give an apparent changing amplitude like for the CVOR, however the receiver perceives a change of frequency due to Doppler shift (as the antennas are "moving").