Supply in the game Decisive Campaigns: Ardennes Offensive

[Judykator]

Can someone explain to me why this unit (243. Stug) is receiving low supplies and zero fuel, even though it's quite close to the depot in Wilitz which I captured, and which has around 3500 supply and 1500 fuel.
Perhaps someone could explain the supply rules in the game more broadly. In particular, whether a unit gets supply from the nearest point or not necessarily, and what influence the player has on all of this. Wiltz is set to 'deplete' mode.