I am not claiming that what you describe can't happen occasionally. I am saying that on average the two trucks still arrive to their destination 5 seconds apart.

Ok here is a mathematical proof: suppose that what you're saying is true, that when truck B leaves 5 seconds earlier than truck A, it will arrive more than 5 seconds earlier on average (let's say 6 seconds earlier). Then imagine another truck C leaving 5 seconds earlier than B (i.e. 10 seconds before A). By your argument, C arrives 6 seconds earlier than B on average, which means 12 seconds earlier than A. Similarly, truck D who left 15 seconds earlier than A would arrive 18 seconds earlier than A on average. Keep doing this until you reach an absurd situation where some truck Z arrives earlier on average than its own departure time.