Different behavior 9g and MG995 , one exact , another crazy . Because?

I Can not entender bought hum servo motor MG995 , thought he portasse equal the 9g , He does not respond to commands as I expected . The 9g and Small To my need , but E misbehave Programming 90 degrees paragraph Right, OR 90 degrees paragraph Left HE performs perfectly What was programmed . The MG995 NOT , HE turns Well More What do requested. NOT AND Exactly How MG995