A reason could be, the owner has oldtimer plates (in Germany we called this 07-plates). You can use the plates for every car in your collection. You need no technical inspection (MOT) and the car must be only roadworthy. The cost are about 200,- Euro tax + 150,- Euro insurance (year).
The 07-plates look like this
The disadvantage is, you can use the 07-registration only for journeys to the car workshop, for test drives and journeys to club meetings.
But a few people ignore this condition and used the plates for cars which they drive regular.
So the age from the cars (you can register) was lifted from 20 to 30 Years last year.
Many cars from 1977 up to the middle ninties you must now registrate regulary. And that costs -> green tax!