The reason for this is that each derailleur has a spring in it that pushes it toward the smallest cog. The long lever on your shifters pulls cable against the tension of this spring to move into bigger cogs, while the short one just releases tension to let the chain go to a smaller cog. You can feel this in their action, the long one is actually doing some work, the short one is just releasing a ratchet. Because a smaller cog is an upshift in the rear and a downshift in the front, they seem to work in different directions.