It will be the manufacturer along with Motability.
If you notice brand new models appear on the scheme for one quarter or even less, depending on sales orders the manufacturer has, they do seem to use the scheme to get cars seen on the roads. For example, the Nissan Aryia was introduced into the scheme in various specs, and now is down to one.
Also, the manufacturers use the scheme to move low selling cars at the end of their lifecycle, these can be good value, as they can be Hugh spec cars. Again, these can be on, then off quickly, as the manufacturers have a volume to be built before changeover and when parts are used up, they cone off sale