Even if you get two of the exact same servo, they are not guaranteed to move exactly the same amount. You can set up your surfaces mechanically to have identical throws, but if the servos are not perfectly matched you will still have uneven surface throws.
You need to understand linearity and monotonicity of devices to understand the problems in matching servos. A simple baseline article is here, but if you become an engineer, you might spend much more time grappling with the subject.
http://www.analogzone.com/nett1108.pdf