Can someone please explain to me why it takes two trucks to execute one polyethylene contract?

I have a batch of trucks but only 14 industrial buildings
- 4 are farms that could only possibly use 1 truck at a time
- 1 is a ranch, again one truck at a time
- oil refinery = 1 truck
- saw mill = 1 truck
- paper mill = 1 truck
- gem mine = 1 truck
That's a total of 9 but the gem mine actually uses a dump truck.

Then I have
- a confectionary plant
- a dairy factory
- milling plant
- weaving factory
- furniture factory
Even if each of these plants used 4 trucks, that's 20 trucks (although some of these trucks used would be milk trucks, wood trucks or fuel trucks) plus the 8 from the others for a total of 28 trucks

But in my garage, I have 37 standard trucks plus a batch of others. I should easily have enough trucks to take care of all my contracts.

Can someone please explain to me the allocation of trucks?