Its because either the hinge line from side to side is different or because the control horns on the control surface are in a different location to each other.. think about it.. if 1mm difference on the control horn adjustments itself for the pushrod make that much of a difference on deflection, then the position of the control horn on the surface will affect it too.\
You can adjust this tho.. if you mix into your radio to use each servo on seperate channels then you can adjust the end points. You need to make the elevator channel the master channel, and the second servo on an aux channel or even a gear/flap channel (make sure you inhibit the switch for gear/flap if you chose that channel). Once you have each channel sorted, you need to mix them in 100% each way so it will be elevator->gear/flap/aux (whichever you chose) then 100% for up and 100% for down.. and make sure you use a mix that allows the master channel to trim both the master and slave when you trim the elevator channel. Then from here you can reverse one channel so they both deflect in the correct direction, and also adjust the end point for the slave channel to match you master channels throw.

I'm not sure if your radio has this capability but check your manual.