What servo's to use depends a lot on what model plane your flying. The bigger the plane the more powerful the servo's will need to be. 3D planes that have large control surfaces with lots of throw will need powerful servo's. Something like a 46 size UCD 3D needs about 90 inch ounces of torque per surface.
Using different servos may be due to people using what they have lying around or because they don't want to waste money on an expensive servo that is 3 time more powerfull than needed. Usually rudder servos are the most powerfull.