The wiper mechanism has got a motor with an axle that turns 360 degrees each time the screen gets completely (going from left to right to left) wiped.
This axle is attached to an offset plate that will turn 360 degrees as well and makes the wiper mechanism rotate 180 degrees to the right and back to the left again.
The offset plate that is attached to the motor's axle determines where the wiper will be parked. If you take out your wiper mechanism and turn the offset plate one or two cogs on the cogged axle, it will be parked at the absolute bottom.
At your car the offset plate is just one (or two) cog(s) out of its original position (for whatever reason). You can change the offset plate into any possible position you want without having to worry that the wiper will rotate to far to the left or right; it can only wipe the windscreen and nothing more...
Here is a picture of the wiper mechanism and the mentioned axle (yellow arrow) with attached offset plate. If you want to have more info I have a page about the wiper mechanism on my personal web site...