I would guess that it's because it's a conversion from a brake van, not purpose-built.
It has something of the look of one of the Thumper
DEMUs▸ about it. I never know if they're "Hastings" or "Tadpoles"
"Hastings" are narrow bodied and typically 6 car sets.
"Tadpoles" are Hastings (narrow) power cars with an intermediate car and a wide bodied EPB drive trailer
"Hamshire" units are 2 and 3 car wide bodied sets based on EPB body shells
"Thumpers" I think of as being the Hampshire units, but probably refers to them all
The trailer shown looks like an EPB based carriage to me