Mortar dude (Subtype Artillery)

I think the answer lies with their ability to be transported by vehicles. Light mortars can be carried in a jeep or kubelwagon because they have the Transport special ability which states that it can carry a single Soldier, not a Soldier - Artillery unit. Larger vehicles such as the Opel Blitz have the SA Large Transport which can carry two Soldier units, one of which can be Artillery.

Light mortars and their ammo are easily carried by infantrymen or a light vehicle. Larger mortars and artillery pieces require more equipment to be transported with them, so trucks would fit that requirement. So the labeling of light mortars as soldiers and heavier artillery & mortars as soldier-artillery solves this issue.