It is designed to be that way.
in these days of 2 people using the car from time to time, it is sort of a Mirror memory function.
what i mean is this -
User 1 gets in car and sets mirrors up. Great.
User 2 gets in car, and uses switch when set on Drivers side to adjust mirror, when the right moves the left will move in the same plane, i.e. you move the mirror out, the left will move in and so on.
Now when User 1 gets back in the car, he sets seat back up, and then adjusts right hand mirror (drivers) back in to his setting, the left moves back out to suit.
It saves having to set up both mirrors. Pretty simple idea but clever stuff.