MD-8Xs elevator

[B767PL]

Everyday I see MD-8Xs takeoff. As they are rolling onto the runway, or taxing the right elevator is always higher then the left. It seems to go down as they are positioning on the runway. What is the reason for this?? I didnt know they can move independantly of one another.

Its just something thats bugging me because I cant figure out what purpose this serves.

heres a pic :
http://www.!!!!!!!!!!!!!!/open.file/616777/M/
http://www.!!!!!!!!!!!!!!/open.file/383390/M/
http://www.!!!!!!!!!!!!!!/open.file/382601/M/

Those show what im talking about pretty well.