Such narrow gardens are very common in Belgium. The original strip was probably just the width of the house, the strip to the left of the house was then bought at a later date when the buyer could afford it.

It’s just an old field that was divided up in as many buildable plots as possible along an existing road, so the owner of the field can sell the entirety as building terrain, without having to build any amenities. Privatization of the profits, socialization of the costs.