[FROM: Trash Terrier]
Because you could click all wagons except one and then click on the loc and still get the reward for the complete train.
So in this way you got almost double of the real reward.
I've noticed this too, though i have to disappoint you, it was only visual
Indeed, clicking the loco after unloading one or more wagons showed that it counted the unloaded wagons in the totals, but when you've checked the delivered mats, they were just what you've delivered (+ unload bonus)
I've tried to complete a contract this way, it didn't work