acceleroemeter adxl335

Hi , I'm trying to program the accelerometer " ADXL335 " for acceleration values ​​, the purpose is to measure the acceleration of an earthquake ; the issue is that I found codes to program the accelerometer , and apparently are codes that give me degrees of slope or sensitivity , ( I need in measures g ) , the point is that I leave the motionless accelerometer and running the program I consistently delivers different , similar values ​​, but still they are changing, if someone can explain to me why this happens or I'll be using a wrong code?
pd: sorry if the english doesn't understand, i'm working on it :confused:

That is how they behave...